Semiconductor device

ABSTRACT

The semiconductor device includes a first transistor provided in a driver circuit portion and a second transistor provided in a pixel portion; the first transistor and the second transistor have different structures. In an oxide semiconductor film of each of the transistors, an impurity element is contained in regions which do not overlap with a gate electrode. The regions of the oxide semiconductor film which contain the impurity element function as low-resistance regions. Furthermore, the regions of the oxide semiconductor film which contain the impurity element are in contact with a film containing hydrogen. Furthermore, the first transistor provided in the driver circuit portion may include the oxide semiconductor film in which a first film and a second film are stacked, and the second transistor provided in the pixel portion may include the oxide semiconductor film which differs from the first film in the atomic ratio of metal elements.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a semiconductordevice including an oxide semiconductor and a display device includingthe semiconductor device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, the presentinvention relates to a process, a machine, manufacture, or a compositionof matter. In particular, the present invention relates to asemiconductor device, a display device, a light-emitting device, a powerstorage device, a storage device, a driving method thereof, or amanufacturing method thereof.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A semiconductor element such as a transistor, asemiconductor circuit, an arithmetic device, and a memory device areeach an embodiment of a semiconductor device. An imaging device, adisplay device, a liquid crystal display device, a light-emittingdevice, an electro-optical device, a power generation device (includinga thin film solar cell, an organic thin film solar cell, and the like),and an electronic device may each include a semiconductor device.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor usinga semiconductor thin film formed over a substrate having an insulatingsurface (also referred to as thin film transistor (TFT)). The transistoris used in a wide range of electronic devices such as an integratedcircuit (IC) or an image display device (display device). Asemiconductor material typified by silicon is widely known as a materialfor a semiconductor thin film that can be used for a transistor. Asanother material, an oxide semiconductor has been attracting attention.

For example, Patent Document 1 discloses a technique in which atransistor is manufactured using an amorphous oxide containing In, Zn,Ga, Sn, and the like as an oxide semiconductor.

REFERENCE Patent Document [Patent Document 1] Japanese Published PatentApplication No. 2006-165529 SUMMARY OF THE INVENTION

As a transistor including an oxide semiconductor film, an invertedstaggered transistor (also referred to as a transistor having abottom-gate structure), a planar transistor (also referred to as atransistor having a top-gate structure), and the like are given. In thecase where a transistor including an oxide semiconductor film is usedfor a display device, an inverted staggered transistor is used moreoften than a planar transistor because a manufacturing process thereofis relatively simple and manufacturing cost thereof can be kept low.However, signal delay or the like is increased by parasitic capacitancethat exists between a gate electrode and source and drain electrodes ofan inverted staggered transistor and accordingly image quality of adisplay device degrades, which has posed a problem, as an increase inscreen size of a display device proceeds, or a display device isprovided with a higher resolution image (for example, a high-resolutiondisplay device typified by 4k×2k pixels (3840 pixels in the horizontaldirection and 2160 pixels in the perpendicular direction) or 8k×4kpixels (7680 pixels in the horizontal direction and 4320 pixels in theperpendicular direction)). Furthermore, as another problem, theoccupation area of an inverted staggered transistor is larger than thatof a planar transistor. Thus, with regard to a planar transistorincluding an oxide semiconductor film, development of a transistor whichhas a structure with stable semiconductor characteristics and highreliability and which is formed by a simple manufacturing process isdesired.

In view of the foregoing problems, one embodiment of the presentinvention is to provide a novel semiconductor device including an oxidesemiconductor, particularly to provide a planar type semiconductordevice including an oxide semiconductor. Furthermore, an object is toprovide a semiconductor device including an oxide semiconductor in whichon-state current is high, provide a semiconductor device including anoxide semiconductor in which ofd state current is low, provide asemiconductor device including an oxide semiconductor which occupies asmall area, provide a semiconductor device including an oxidesemiconductor which has stable electrical characteristics, provide asemiconductor device including an oxide semiconductor which has highreliability, provide a novel semiconductor device, or provide a noveldisplay device.

Note that the description of the above object does not disturb theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Objects other than theabove objects will be apparent from and can be derived from thedescription of the specification and the like.

One embodiment of the present invention is a semiconductor deviceincluding a first transistor provided in a driver circuit portion and asecond transistor provided in a pixel portion; the first transistor andthe second transistor have different structures. Furthermore, the firsttransistor and the second transistor are transistors having a top-gatestructure. In an oxide semiconductor film of each of the transistors, animpurity element is contained in regions which do not overlap with agate electrode. The regions of the oxide semiconductor film whichcontain the impurity element function as low-resistance regions.Furthermore, the regions of the oxide semiconductor film which containthe impurity element are in contact with a film containing hydrogen. Inaddition, conductive films functioning as a source electrode and a drainelectrode which are in contact with the regions containing the impurityelement through openings in the film containing hydrogen may beprovided.

One embodiment of the present invention is a semiconductor deviceincluding a first transistor provided in a driver circuit portion and asecond transistor and a third transistor provided in a pixel portion; atleast the second transistor and the third transistor have differentstructures. Furthermore, the first transistor, the second transistor,and the third transistor are transistors having a top-gate structure. Inan oxide semiconductor film of each of the transistors, an impurityelement is contained in regions which do not overlap with a gateelectrode. Furthermore, the regions of the oxide semiconductor filmwhich contain the impurity element are in contact with a film containinghydrogen. In addition, conductive films functioning as a sourceelectrode and a drain electrode which are in contact with the regionscontaining the impurity element through openings in the film containinghydrogen may be provided.

Note that the first transistor provided in the driver circuit portionmay include two gate electrodes overlapping with each other with theoxide semiconductor film provided therebetween.

Note that the first transistor provided in the driver circuit portionand the third transistor provided in the pixel portion may each includetwo gate electrodes overlapping with each other with the oxidesemiconductor film provided therebetween.

Furthermore, the first transistor provided in the driver circuit portionmay include the oxide semiconductor film in which a first film and asecond film are stacked, and the second transistor provided in the pixelportion may include the oxide semiconductor film which differs from thefirst film in the atomic ratio of metal elements. Furthermore, in theoxide semiconductor film included in the second transistor, the atomicratio of metal elements may be the same as the atomic ratio of metalelements of the second film included in the oxide semiconductor film ofthe first transistor.

Furthermore, the third transistor provided in the pixel portion mayinclude the oxide semiconductor film in which a first film and a secondfilm are stacked, and the second transistor provided in the pixelportion may include the oxide semiconductor film which differs from thefirst film in the atomic ratio of metal elements. Furthermore, in theoxide semiconductor film included in the second transistor, the atomicratio of metal elements may be the same as the atomic ratio of metalelements of the second film included in the oxide semiconductor film ofthe third transistor.

Furthermore, the first transistor provided in the driver circuit portionand the third transistor provided in the pixel portion may each includethe oxide semiconductor film in which a first film and a second film arestacked, and the first film and the second film may have differentatomic ratios of metal elements.

As the impurity element, hydrogen, boron, carbon, nitrogen, fluorine,aluminum, silicon, phosphorus, chlorine, or a rare gas element is given.

When containing hydrogen and at least one of a rare gas element, boron,carbon, nitrogen, fluorine, aluminum, silicon, phosphorus, and chlorineas an impurity element, the oxide semiconductor film has higherconductivity. Thus, when regions containing the impurity element areprovided in a region which does not overlap with the gate electrode inthe oxide semiconductor film and the regions containing the impurityelement are in contact with the source electrode and the drainelectrode, the parasitic resistance and parasitic capacitance of thetransistor can be reduced, and the transistor having high on-statecurrent is obtained.

One embodiment of the present invention can provide a novelsemiconductor device including an oxide semiconductor. In particular, aplanar type semiconductor device including an oxide semiconductor can beprovided. Alternatively, a semiconductor device including an oxidesemiconductor in which on-state current is high can be provided, asemiconductor device including an oxide semiconductor in which off-statecurrent is low can be provided, a semiconductor device including anoxide semiconductor which occupies a small area can be provided, asemiconductor device including an oxide semiconductor which has stableelectrical characteristics can be provided, a semiconductor deviceincluding an oxide semiconductor which has high reliability can beprovided, a novel semiconductor device can be provided, or a noveldisplay device can be provided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are top views illustrating one embodiment of asemiconductor device.

FIGS. 2A and 2B are cross-sectional views illustrating one embodiment ofa semiconductor device.

FIGS. 3A and 3B are cross-sectional views illustrating one embodiment ofa semiconductor device.

FIGS. 4A to 4C are cross-sectional views illustrating one embodiment ofa method for manufacturing a semiconductor device.

FIGS. 5A to 5C are cross-sectional views illustrating one embodiment ofa method for manufacturing a semiconductor device.

FIGS. 6A and 6B are cross-sectional views illustrating one embodiment ofa method for manufacturing a semiconductor device.

FIGS. 7A and 7B are top views illustrating one embodiment of asemiconductor device.

FIGS. 8A and 8B are cross-sectional views illustrating one embodiment ofa semiconductor device.

FIGS. 9A and 9B are cross-sectional views illustrating one embodiment ofa semiconductor device.

FIGS. 10A and 10B are cross-sectional views each illustrating oneembodiment of a semiconductor device.

FIGS. 11A to 11C are top views illustrating one embodiment of asemiconductor device.

FIG. 12 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIG. 13 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIG. 14 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIG. 15 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIG. 16 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIG. 17 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIGS. 18A to 18C are top views illustrating one embodiment of asemiconductor device.

FIG. 19 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIG. 20 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIGS. 21A to 21C are top views illustrating one embodiment of asemiconductor device.

FIG. 22 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIG. 23 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIGS. 24A and 24B are cross-sectional views illustrating one embodimentof a method for manufacturing a semiconductor device.

FIGS. 25A and 25B are cross-sectional views illustrating one embodimentof a method for manufacturing a semiconductor device.

FIGS. 26A and 26B are cross-sectional views illustrating one embodimentof a method for manufacturing a semiconductor device.

FIGS. 27A and 27B are cross-sectional views illustrating one embodimentof a method for manufacturing a semiconductor device.

FIGS. 28A and 28B show band diagrams of a transistor of one embodimentof the present invention.

FIGS. 29A to 29F are cross-sectional views each illustrating a structureof a transistor.

FIGS. 30A to 30F are cross-sectional views each illustrating a structureof a transistor.

FIGS. 31A to 31E are cross-sectional views each illustrating a structureof a transistor.

FIGS. 32A and 32B are cross-sectional views each illustrating astructure of a transistor.

FIGS. 33A to 33D are cross-sectional views each illustrating a structureof a transistor.

FIGS. 34A and 34B are cross-sectional views illustrating a manufacturingprocess of transistors.

FIG. 35 shows a calculation model.

FIGS. 36A and 36B show an initial state and a final state, respectively.

FIG. 37 shows an activation barrier.

FIGS. 38A and 38B show an initial state and a final state, respectively.

FIG. 39 shows an activation barrier.

FIG. 40 shows the transition levels of V_(O)H.

FIGS. 41A to 41D are a block diagram and circuit diagrams illustrating adisplay device.

FIG. 42 is a top view illustrating one embodiment of a display device.

FIGS. 43A and 43B are cross-sectional views each illustrating oneembodiment of a display device.

FIGS. 44A and 44B are cross-sectional views each illustrating oneembodiment of a display device.

FIG. 45 is a cross-sectional view illustrating a structure of a pixelportion of a light-emitting device.

FIG. 46 illustrates a display module.

FIGS. 47A to 47G illustrate electronic devices.

FIG. 48 shows temperature dependence of resistivity.

FIGS. 49A to 49D are Cs-corrected high-resolution TEM images of a crosssection of a CAAC-OS and a cross-sectional schematic view of a CAAC-OS.

FIGS. 50A to 50D are Cs-corrected high-resolution TEM images of a planeof a CAAC-OS.

FIGS. 51A to 51C show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor by XRD.

FIGS. 52A and 52B show electron diffraction patterns of a CAAC-OS.

FIG. 53 shows a change in crystal part of an In—Ga—Zn oxide induced byelectron irradiation.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the invention disclosed in thisspecification will be described with reference to the accompanyingdrawings. Note that the present invention is not limited to thefollowing description and it will be readily appreciated by thoseskilled in the art that modes and details can be modified in variousways without departing from the spirit and the scope of the presentinvention. Accordingly, the present invention should not be interpretedas being limited to the content of the embodiments below.

Note that the position, the size, the range, or the like of eachstructure illustrated in drawings and the like is not accuratelyrepresented in some cases for simplification. Therefore, the disclosedinvention is not necessarily limited to the position, the size, therange, or the like disclosed in the drawings and the like.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not mean limitation of the number ofcomponents.

Note that the term such as “over” or “below” in this specification andthe like does not necessarily mean that a component is placed “directlyon” or “directly under” another component. For example, the expression“a gate electrode over a gate insulating film” can mean the case wherethere is an additional component between the gate insulating film andthe gate electrode.

In addition, in this specification and the like, the term such as an“electrode” or a “wiring” does not limit a function of a component. Forexample, an “electrode” is used as part of a “wiring” in some cases, andvice versa. Further, the to m “electrode” or “wiring” can also mean acombination of a plurality of “electrodes” and “wirings” formed in anintegrated manner.

Functions of a “source” and a “drain” are sometimes replaced with eachother when a transistor of opposite polarity is used or when thedirection of flow of current is changed in circuit operation, forexample. Therefore, the terms “source” and “drain” can be used to denotethe drain and the source, respectively, in this specification and thelike.

Note that in this specification and the like, the term “electricallyconnected” includes the case where components are connected through anobject having any electric function. There is no particular limitationon an “object having any electric function” as long as electric signalscan be transmitted and received between components that are connectedthrough the object. Examples of an “object having any electric function”are a switching element such as a transistor, a resistor, an inductor, acapacitor, and elements with a variety of functions as well as anelectrode and a wiring.

Embodiment 1

In this embodiment, one embodiment of a semiconductor device and amethod for manufacturing the semiconductor device will be described withreference to FIGS. 1A and 1B, FIGS. 2A and 2B, FIGS. 3A and 3B, FIGS. 4Ato 4C, FIGS. 5A to 5C, FIGS. 6A and 6B, FIGS. 7A and 7B, FIGS. 8A and8B, FIGS. 9A and 9B, and FIGS. 10A and 10B.

<Structure 1 of Semiconductor Device>

In FIGS. 1A and 1B and FIGS. 2A and 2B, transistors each having atop-gate structure are shown as examples of transistors included in asemiconductor device. Here, a display device is described as an exampleof the semiconductor device. Furthermore, structures of transistorsprovided in a driver circuit portion and a pixel portion of the displaydevice are described. In this embodiment, the transistor provided in thedriver circuit portion differs from the transistor provided in the pixelportion in the structure of an oxide semiconductor film.

FIGS. 1A and 1B are top views of a transistor 100 g provided in a drivercircuit portion and a transistor 100 h provided in a pixel portion.FIGS. 2A and 2B are cross-sectional views of the transistors 100 g and100 h. FIG. 1A is the top view of the transistor 100 g, and FIG. 1B isthe top view of the transistor 100 h. FIG. 2A shows cross-sectionalviews along the dashed-dotted line A-B in FIG. 1A and the dashed-dottedline C-D in FIG. 1B. FIG. 2B shows cross-sectional views along thedashed-dotted line G-H in FIG. 1A and the dashed-dotted line I-J in FIG.1B. Note that in FIGS. 1A and 1B, a substrate 101, an insulating film104, an insulating film 126, an insulating film 127, and the like arenot illustrated for simplicity. FIG. 2A shows cross-sectional views ofthe transistors 100 g and 100 h in a channel length direction, and FIG.2B shows cross-sectional views of the transistors 100 g and 100 h in achannel width direction.

In a manner similar to that of the transistors 100 g and 100 h, somecomponents are not illustrated in some cases in top views of transistorsdescribed below. Furthermore, the direction of the dashed-dotted lineA-B and the direction of the dashed-dotted line C-D may be called achannel length direction, and the direction of the dashed-dotted lineG-H and the direction of the dashed-dotted line I-J may be called achannel width direction.

The transistor 100 g shown in FIGS. 2A and 2B includes a multilayer film107 over the insulating film 104 formed over the substrate 101; aninsulating film 116 in contact with the multilayer film 107; and aconductive film 119 overlapping with the multilayer film 107 with theinsulating film 116 provided therebetween. The conductive film 119functions as a gate electrode. The insulating film 116 functions as agate insulating film. The multilayer film 107 includes a channel region107 a and low-resistance regions 107 b and 107 c. The channel region 107a includes a channel region 105 a in contact with the insulating film104 and a channel region 106 a in contact with the channel region 105 a.The low-resistance region 107 b includes a low-resistance region 105 bin contact with the insulating film 104 and a low-resistance region 106b in contact with the low-resistance region 105 b. The low-resistanceregion 107 c includes a low-resistance region 105 c in contact with theinsulating film 104 and a low-resistance region 106 c in contact withthe low-resistance region 105 c. Note that although not shown in FIGS.2A and 2B, an oxide semiconductor film including the channel region 105a, the low-resistance region 105 b, and the low-resistance region 105 cis referred to as an oxide semiconductor film 105, and an oxidesemiconductor film including the channel region 106 a, thelow-resistance region 106 b, and the low-resistance region 106 c isreferred to as an oxide semiconductor film 106. That is, the multilayerfilm 107 is a stack including the oxide semiconductor film 105 and theoxide semiconductor film 106.

Note that in a top surface shape, an edge portion of the oxidesemiconductor film 106 is positioned outside an edge portion of theoxide semiconductor film 105. That is, the oxide semiconductor film 106covers a top surface and a side surface of the oxide semiconductor film105.

In the transistor 100 g, the insulating film 126 in contact with thelow-resistance regions 107 b and 107 c is provided. Furthermore, theinsulating film 127 may be provided over the insulating film 126, Inaddition, conductive films 134 and 135 which are in contact with thelow-resistance regions 107 b and 107 c of the multilayer film 107through openings 128 and 129 in the insulating films 126 and 127 areprovided.

The transistor 100 h includes an oxide semiconductor film 108 over theinsulating film 104 formed over the substrate 101; an insulating film117 in contact with the oxide semiconductor film 108; and a conductivefilm 120 overlapping with the oxide semiconductor film 108 with theinsulating film 117 provided therebetween.

The conductive film 120 functions as a gate electrode. The insulatingfilm 117 functions as a gate insulating film.

The oxide semiconductor film 108 includes a channel region 108 aoverlapping with the conductive film 120 and low-resistance regions 108b and 108 c between which the channel region 108 a is positioned.

In the transistor 100 h, the insulating film 126 in contact with thelow-resistance regions 108 b and 108 c is provided. Furthermore, theinsulating film 127 may be provided over the insulating film 126. Inaddition, conductive films 136 and 137 which are in contact with thelow-resistance regions 108 b and 108 c of the oxide semiconductor film108 through openings 130 and 131 in the insulating films 126 and 127 areprovided.

Note that a nitride insulating film 162 is preferably provided to coverthe conductive films 134, 135, 136, and 137, The nitride insulating film162 can prevent diffusion of impurities from the outside.

In the transistors 100 g and 100 h, the oxide semiconductor film 105included in the multilayer film 107 and the oxide semiconductor film 108have different compositions, whereas the oxide semiconductor film 106included in the multilayer film 107 and the oxide semiconductor film 108have the same composition. That is, the oxide semiconductor film 105 isformed in a step different from the step of forming the oxidesemiconductor film 108, and, in addition, the oxide semiconductor film106 and the oxide semiconductor film 108 are formed in the same step.

A channel of the transistor 100 g is formed in the oxide semiconductorfilm 105. Accordingly, the oxide semiconductor film 105 has a largerthickness than the oxide semiconductor film 106.

The thickness of the oxide semiconductor film 105 is greater than orequal to 3 nm and less than or equal to 200 nm, greater than or equal to10 nm and less than or equal to 50 nm, or greater than or equal to 20 nmand less than or equal to 35 nm. The thickness of each of the oxidesemiconductor films 106 and 108 is greater than or equal to 3 nm andless than or equal to 200 nm, greater than or equal to 3 nm and lessthan or equal to 100 nm, greater than or equal to 10 nm and less than orequal to 100 run, or greater than or equal to 30 nm and less than orequal to 50 nm.

The oxide semiconductor films 105, 106, and 108 are each formed using ametal oxide containing at least In, and typically formed using an In—Gaoxide, an In-M-Zn oxide (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf),or the like. The oxide semiconductor film 105 has a higher indiumcontent than the oxide semiconductor film 106; therefore, a buriedchannel can be formed in the transistor 100 g. Thus, variations in thethreshold voltage of the transistor 100 g can be reduced and channelresistance can be lowered. The details are described in <Band Structure>below.

In the oxide semiconductor film 105, the proportion of n atoms is higherthan that of M (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf) atoms. Inthe case where the oxide semiconductor film 105 contains an 1 n-M-Znoxide (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce, Nd, or HO, and a targethaving the atomic ratio of the metal elements of In:M:Zn x₁:y₁:z₁ isused for forming the oxide semiconductor film 105, x₁/y₁ is preferablygreater than 1 and less than or equal to 6. Typical examples of theatomic ratio of the metal elements of the target are In:M:Zn=2:1:1.5,In:M:Zn=2:1:23, In:M:Zn=2:1:3, In:M:Zn=3:1:2, In:M:Zn=3:1:3, andIn:M:Zn=3:1:4.

In the oxide semiconductor films 106 and 108, the proportion of In atomsis lower than or equal to that of M (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce,Nd, or Hf) atoms. In the case where the oxide semiconductor films 106and 108 contain an In-M-Zn oxide (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce,Nd, or Hf), and a target having the atomic ratio of the metal elementsof In:M:Zn=x₂:y₂:z₂ is used for forming the oxide semiconductor films106 and 108, x₂/y₂ is preferably greater than or equal to ⅙ and lessthan or equal to 1, and z₂/y₂ is preferably greater than or equal to ⅓and less than or equal to 6, further preferably greater than or equal to1 and less than or equal to 6. Note that when z₂/y₂ is greater than orequal to 1 and less than or equal to 6, c-axis aligned crystalline oxidesemiconductor (CAAC-OS) films are easily formed as the oxidesemiconductor films 106 and 108. Typical examples of the atomic ratio ofthe metal elements of the target are In:M:Zn=1:1:1, In:M:Zn=1:1:1.2,In:M:Zn=1:3:2, In:M:Zn=1:3:4, In:M:Zn=1:3:6, In:M:Zn=1:3:8,In:M:Zn=1:4:4, In:M:Zn=1:4:5, In:M:Zn=1:4:6, In:M:Zn=In:M:Zn=1:4:8,In:M:Zn=1:5:5, In:M:Zn=1:5:6, In:M:Zn=1:5:7, In:M:Zn=1:5:8, andIn:M:Zn=1:6:8.

The transistor 100 g has high field-effect mobility because a channel isformed in the oxide semiconductor film 105 in which the proportion of Inatoms is higher than that of M (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce, Nd,or HO atoms. Typically, the transistor has a field-effect mobility ofgreater than 10 cm²/V·s and less than 60 cm²/V·s, preferably greaterthan or equal to 15 cm²/V·s and less than 50 cm²N·s. However, theoff-state current of the transistor is increased due to lightirradiation. Therefore, by providing a light-blocking film in the drivercircuit portion, a transistor with high field-effect mobility and lowoff-state current is obtained. Consequently, a driver circuit portioncapable of high-speed operation can be formed.

In the transistor 100 h, a channel is formed in the oxide semiconductorfilm in which the proportion of In atoms is lower than or equal to thatof M (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf) atoms. Thus, evenwhen the oxide semiconductor film is irradiated with light, the amountof increase in off-state current is small. Therefore, by providing thetransistor including the oxide semiconductor film in which theproportion of In atoms is lower than or equal to that of M (M is Mg, Al,Ti, Ga, Y, Zr, La, Ce, Nd, or Hf) atoms in the pixel portion, the pixelportion that hardly deteriorates due to light irradiation and provideshigh display quality can be obtained.

Furthermore, in the display device, the transistor included in thedriver circuit portion and the transistor included in the pixel portionmay have different channel lengths.

Typically, the channel length of the transistor 100 g included in thedriver circuit portion can be less than 2.5 μm, or greater than or equalto 1.45 μm and less than or equal to 2.2 μm. The channel length of thetransistor 100 h included in the pixel portion can be greater than orequal to 2.5 μm, or greater than or equal to 2.5 μm and less than orequal to 20 μm.

When the channel length of the transistor 100 g included in the drivercircuit portion is less than 2.5 μm, preferably greater than or equal to1.45 μm and less than or equal to 2.2 μm, as compared with thetransistor 100 h included in the pixel portion, the field-effectmobility can be increased, and the amount of on-state current can beincreased. Consequently, a driver circuit portion capable of high-speedoperation can be formed.

In the multilayer film 107, an element which forms an oxygen vacancy isincluded in a region which does not overlap with the conductive film119. In the oxide semiconductor film 108, an element which forms anoxygen vacancy is included in a region which does not overlap with theconductive film 120. Hereinafter, elements which form oxygen vacanciesin an oxide semiconductor film by being added thereto are described asimpurity elements. Typical examples of impurity elements are hydrogen,boron, carbon, nitrogen, fluorine, aluminum, silicon, phosphorus,chlorine, and rare gas elements. Typical examples of rare gas elementsare helium, neon, argon, krypton, and xenon.

The insulating film 126 is a film containing hydrogen, and a nitrideinsulating film is a typical example thereof. Examples of a nitrideinsulating film include a silicon nitride film and an aluminum nitridefilm. The insulating film 126 is in contact with the multilayer film 107and the oxide semiconductor film 108. Therefore, hydrogen contained inthe insulating film 126 is diffused into the multilayer film 107 and theoxide semiconductor film 108. As a result, much hydrogen is contained ina region in contact with the insulating film 126 in the multilayer film107 and the oxide semiconductor film 108.

When the impurity element is added to the oxide semiconductor, a bondbetween a metal element and oxygen in the oxide semiconductor is cut,whereby an oxygen vacancy is formed. When hydrogen is added to an oxidesemiconductor in which an oxygen vacancy is formed by addition of animpurity element, hydrogen enters an oxygen vacant site and forms adonor level in the vicinity of the conduction band; thus, theconductivity of the oxide semiconductor is increased. Consequently, anoxide conductor can be formed. Accordingly, the oxide conductor has alight-transmitting property. Here, an oxide conductor refers to an oxidesemiconductor having become a conductor.

The oxide conductor is a degenerate semiconductor and it is suggestedthat the conduction band edge equals to or substantially equals to theFermi level. For that reason, an ohmic contact is made between an oxideconductor film and conductive films functioning as a source electrodeand a drain electrode; thus, contact resistance between the oxideconductor film and the conductive films functioning as a sourceelectrode and a drain electrode can be reduced.

In other words, the low-resistance regions 107 b, 107 c, 108 b, and 108c function as source regions and drain regions.

In the case where the conductive films 134, 135, 136, and 137 are formedusing a conductive material which is easily bonded to oxygen, such astungsten, titanium, aluminum, copper, molybdenum, chromium, tantalum, analloy of any of these, or the like, oxygen contained in the oxidesemiconductor films is bonded to the conductive material contained inthe conductive films 134, 135, 136, and 137, and an oxygen vacancy isformed in the multilayer film 107 and the oxide semiconductor film 108.Furthermore, in some cases, part of constituent elements of theconductive material that forms the conductive films 134, 135, 136, and137 is mixed into the multilayer film 107 and the oxide semiconductorfilm 108. As a result, the low-resistance regions 107 b, 107 c, 108 b,and 108 c in contact with the conductive films 134, 135, 136, and 137have higher conductivity and function as source regions and drainregions.

In the case where the impurity element is a rare gas element and themultilayer film 107 and the oxide semiconductor film 108 are formed by asputtering method, the low-resistance regions 107 b, 107 c, 108 b, and108 c each contain a rare gas element. In addition, the rare gas elementconcentrations of the low-resistance regions 107 b, 107 c, 108 b, and108 c are higher than those of the channel regions 107 a and 108 a. Thereasons are as follows: in the case where the multilayer film 107 andthe oxide semiconductor film 108 are formed by a sputtering method, arare gas is used as a sputtering gas, so that the multilayer film 107and the oxide semiconductor film 108 contain the rare gas; and a raregas is intentionally added to the low-resistance regions 107 b, 107 c,108 b, and 108 c in order to form oxygen vacancies in the low-resistanceregions 107 b, 107 c, 108 b, and 108 c. Note that a rare gas elementdifferent from that added to the channel regions 107 a and 108 a may beadded to the low-resistance regions 107 b, 107 c, 108 b, and 108 c.

Since the low-resistance regions 107 b and 107 c are in contact with theinsulating film 126, the concentration of hydrogen in the low-resistanceregions 107 b and 107 c is higher than the concentration of hydrogen inthe channel region 107 a. In addition, since the low-resistance regions108 b and 108 c are in contact with the insulating film 126, theconcentration of hydrogen in the low-resistance regions 108 b and 108 cis higher than the concentration of hydrogen in the channel region 108a.

In the low-resistance regions 107 b, 107 c, 108 b, and 108 c, theconcentrations of hydrogen which are measured by secondary ion massspectrometry (SIMS) can be higher than or equal to 8×10¹⁹ atoms/cm³,higher than or equal to 1×10²⁰ atoms/cm³, or higher than or equal to5×10²⁰ atoms/cm³. Note that in the channel regions 107 a and 108 a, theconcentrations of hydrogen which are measured by SIMS can be lower thanor equal to 5×10¹⁹ atoms/cm³, lower than or equal to 1×10¹⁹ atoms/cm³,lower than or equal to 5×10¹⁸ atoms/cm³, lower than or equal to 1×10¹⁸atoms/cm³, lower than or equal to 5×10¹⁷ atoms/cm³, or lower than orequal to 1×10¹⁶ atoms/cm³.

The low-resistance regions 107 b, 107 c, 108 b, and 108 c have higherhydrogen concentrations than the channel regions 107 a and 108 a andhave more oxygen vacancies than the channel regions 107 a and 108 abecause of addition of rare gas elements. Therefore, the low-resistanceregions 107 b, 107 c, 108 b, and 108 c have higher conductivity andfunction as source regions and drain regions. The resistivity of thelow-resistance regions 107 b, 107 c, 108 b, and 108 c can be typicallygreater than or equal to 1×10⁻³ Ωcm and less than 1×10⁴ Ωcm, or greaterthan or equal to 1×10⁻³ Ωcm and less than 1×10⁻¹ Ωcm.

Note that in the low-resistance regions 107 b, 107 c, 108 b, and 108 c,when the amount of hydrogen is smaller than or equal to the amount ofoxygen vacancy, hydrogen is easily captured by the oxygen vacancy and isnot easily diffused into the channel regions 107 a and 108 a. As aresult, normally-off transistors can be manufactured.

Furthermore, in the case where the amount of oxygen vacancy is largerthan the amount of hydrogen in the low-resistance regions 107 b, 107 c,108 b, and 108 c, the carrier density of the low-resistance regions 107b, 107 c, 108 b, and 108 c can be controlled by controlling the amountof hydrogen. Alternatively, in the case where the amount of hydrogen islarger than the amount of oxygen vacancy in the low-resistance regions107 b, 107 c, 108 b, and 108 e, the carrier density of thelow-resistance regions 107 b, 107 c, 108 b, and 108 c can be controlledby controlling the amount of oxygen vacancy. Note that when the carrierdensity of the low-resistance regions 107 b, 107 c, 108 b, and 108 c isgreater than or equal to 5×10¹⁸/cm³, greater than or equal to1×10¹⁹/cm³, or greater than or equal to 1×10²⁰/cm³, in the transistors,the resistance between the channel region 107 a and the conductive films134 and 135 functioning as source and drain electrodes and between thechannel region 108 a and the conductive films 136 and 137 functioning assource and drain electrodes is small and high on-state current can beobtained.

In the transistors 100 g and 100 h described in this embodiment, thelow-resistance regions 107 b and 107 c are provided between the channelregion 107 a and the conductive films 134 and 135 functioning as sourceand drain electrodes, and the low-resistance regions 108 b and 108 c areprovided between the channel region 108 a and the conductive turns 136and 137 functioning as source and drain electrodes; therefore, thetransistors have small parasitic resistance.

Furthermore, in the transistor 100 g, the conductive film 119 does notoverlap with the conductive films 134 and 135; therefore, parasiticcapacitance between the conductive film 119 and each of the conductivefilms 134 and 135 can be reduced. In the transistor 100 h, theconductive film 120 does not overlap with the conductive films 136 and137; therefore, parasitic capacitance between the conductive film 120and each of the conductive films 136 and 137 can be reduced. As aresult, in the case where a large-area substrate is used as thesubstrate 101, signal delay in the conductive films 119, 120, 134, 135,136, and 137 can be reduced.

Consequently, the transistors 100 g and 100 h have high on-state currentand high field-effect mobility.

In the transistor 100 g, the impurity element is added to the multilayerfilm 107 using the conductive film 119 as a mask. In the transistor 100h, the impurity element is added to the oxide semiconductor film 108using the conductive film 120 as a mask. That is, the low-resistanceregions can be formed in a self-aligned manner.

The transistor 100 g in the driver circuit portion has high on-statecurrent and high field-effect mobility. Therefore, a display device inwhich the area occupied by a driver circuit portion is small can bemanufactured.

By using the transistor with high field-effect mobility, a demultiplexercircuit can be formed in a signal line driver circuit which is anexample of the driver circuit portion. A demultiplexer circuitdistributes one input signal to a plurality of outputs; thus, using thedemultiplexer circuit can reduce the number of input terminals for inputsignals. For example, when one pixel includes a red sub-pixel, a greensub-pixel, and a blue sub-pixel and a demultiplexer circuitcorresponding to each pixel is provided, an input signal can bedistributed by the demultiplexer circuit to be input to each sub-pixel.Consequently, the number of input terminals can be reduced to ⅓.

The transistor 100 h having high on-state current is provided in thepixel portion; thus, signal delay in wirings can be reduced and displayunevenness can be suppressed even in a large-sized display device or ahigh-resolution display device in which the number of wirings isincreased.

As described above, when a driver circuit portion is formed using atransistor capable of high-speed operation and a pixel portion is formedusing a transistor with small parasitic capacitance and small parasiticresistance, a high-resolution display device capable of double-framerate driving can be manufactured.

The structure shown in FIGS. 1A and 1B is described in detail below.

As the substrate 101, any of a variety of substrates can be used withoutparticular limitation. Examples of the substrate include a semiconductorsubstrate (e.g., a single crystal substrate or a silicon substrate), anSOI substrate, a glass substrate, a quartz substrate, a plasticsubstrate, a metal substrate, a stainless steel substrate, a substrateincluding stainless steel foil, a tungsten substrate, a substrateincluding tungsten foil, a flexible substrate, an attachment film, paperincluding a fibrous material, and a base material film. Examples of theglass substrate are a barium borosilicate glass substrate, analuminoborosilicate glass substrate, and a soda lime glass substrate.Examples of a flexible substrate, an attachment film, a base materialfilm, or the like are as follows: plastic typified by polyethyleneterephthalate (PET), polyethylene naphthalate (PEN), and polyethersulfone (PES); a synthetic resin such as acrylic; polypropylene;polyvinyl fluoride; polyvinyl chloride; polyester; polyamide; polyimide;aramid; epoxy; an inorganic vapor deposition film; and paper.Specifically, when a transistor is formed using a semiconductorsubstrate, a single crystal substrate, an SOI substrate, or the like, itis possible to form a transistor with few variations in characteristics,size, shape, or the like, with high current supply capability, and witha small size. By forming a circuit with the use of such a transistor,power consumption of the circuit can be reduced or the circuit can behighly integrated.

A flexible substrate may be used as the substrate 101, and thetransistors may be provided directly on the flexible substrate.Alternatively, a separation layer may be provided between the substrate101 and each of the transistors. The separation layer can be used whenpart or the whole of a semiconductor device formed over the separationlayer is separated from the substrate 101 and transferred onto anothersubstrate. In such a case, the transistors can be transferred to asubstrate having low heat resistance or a flexible substrate as well.For the above separation layer, a stack including inorganic films, whichare a tungsten film and a silicon oxide film, or an organic resin filmof polyimide or the like formed over a substrate can be used, forexample.

Examples of a substrate to which the transistors are transferredinclude, in addition to the above-described substrates over whichtransistors can be formed, a paper substrate, a cellophane substrate, anaramid film substrate, a polyimide film substrate, a stone substrate, awood substrate, a cloth substrate (including a natural fiber (e.g.,silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, orpolyester), a regenerated fiber (e.g., acetate, cupra, rayon, orregenerated polyester), or the like), a leather substrate, a rubbersubstrate, and the like. When such a substrate is used, a transistorwith excellent properties or a transistor with low power consumption canbe formed, a device with high durability or high heat resistance can beprovided, or reduction in weight or thickness can be achieved.

The insulating film 104 can be formed with a single layer or stackedlayers using an oxide insulating film or a nitride insulating film. Notethat at least regions of the insulating film 104 which are in contactwith the multilayer film 107 and the oxide semiconductor film 108 arepreferably formed using an oxide insulating film, in order to improvecharacteristics of the interface with the multilayer film 107 and theoxide semiconductor film 108. When the insulating film 104 is formedusing an oxide insulating film from which oxygen is released by heating,oxygen contained in the insulating film 104 can be moved to themultilayer film 107 and the oxide semiconductor film 108 by heattreatment. A region of the insulating film 104 which is in contact withthe substrate 101 is preferably forming using a nitride insulating film,in which case elements contained in the substrate 101 can be preventedfrom moving to the multilayer film 107 and the oxide semiconductor film108.

The thickness of the insulating film 104 can be greater than or equal to50 nm, greater than or equal to 100 nm and less than or equal to 3000nm, or greater than or equal to 200 nm and less than or equal to 1000nm. By increasing the thickness of the insulating film 104, the amountof oxygen released from the insulating film 104 can be increased, andthe interface state density at the interface between the insulating film104 and each of the multilayer film 107 and the oxide semiconductor film108 and oxygen vacancies contained in the channel region 107 a in themultilayer film 107 and the channel region 108 a in the oxidesemiconductor film 108 can be reduced.

The insulating film 104 may be formed with a single layer or stackedlayers using one or more of silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, aluminum oxide, hafnium oxide, galliumoxide, a Ga—Zn oxide, and the like.

Here, the insulating film 104 is formed by stacking insulating films 104a and 104 b. When a nitride insulating film is used as the insulatingfilm 104 a, diffusion of elements contained in the substrate 101 can beprevented. When an oxide insulating film is used as the insulating film104 b, the interface state density at the interface between theinsulating film 104 and each of the multilayer film 107 and the oxidesemiconductor film 108 can be reduced, for example.

When silicon or carbon that is one of elements belonging to Group 14 iscontained in the multilayer film 107 and the oxide semiconductor film108, oxygen vacancies are increased in the multilayer film 107 and theoxide semiconductor film 108, and the multilayer film 107 and the oxidesemiconductor film 108 become n-type films. Thus, the concentrations ofsilicon or carbon (the concentration measured by SIMS) in the multilayerfilm 107 and the oxide semiconductor film 108, in particular, thechannel regions 107 a and 108 a, can be set to lower than or equal to2×10¹⁸ atoms/cm³, or lower than or equal to 2×10¹⁷ atoms/cm³. As aresult, the transistors each have positive threshold voltage(normally-off characteristics).

Furthermore, the concentrations of alkali metal or alkaline earth metalwhich are measured by SIMS in the multilayer film 107 and the oxidesemiconductor film 108, in particular, the channel regions 107 a and 108a, can be lower than or equal to 1×10¹⁸ atoms/cm³, or lower than orequal to 2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal mightgenerate carriers when bonded to an oxide semiconductor, in which casethe off-state current of the transistors might be increased. Therefore,it is preferable to reduce the concentrations of alkali metal oralkaline earth metal in the channel regions 107 a and 108 a. As aresult, the transistors each have positive threshold voltage(normally-off characteristics).

Furthermore, when nitrogen is contained in the multilayer film 107 andthe oxide semiconductor film 108, in particular, the channel regions 107a and 108 a, electrons serving as carriers are generated, carrierdensity is increased, and the multilayer film 107 and the oxidesemiconductor film 108 become n-type films in some cases. Thus, atransistor including an oxide semiconductor film which contains nitrogenis likely to have normally-on characteristics. Therefore, nitrogen ispreferably reduced as much as possible in the oxide semiconductor films,in particular, the channel regions 107 a and 108 a. The concentrationsof nitrogen which are measured by SIMS can be set to, for example, lowerthan or equal to 5×10¹⁸ atoms/cm³.

By reducing the impurity elements in the multilayer film 107 and theoxide semiconductor film 108, in particular, the channel regions 107 aand 108 a, the carrier density of the oxide semiconductor films can belowered. In the multilayer film 107 and the oxide semiconductor film108, in particular, the channel regions 107 a and 108 a, carrier densitycan be set to 1×10¹⁷/cm³ or less, 1×10¹⁵/cm³ or less, 1×10¹³/cm³ orless, 8×10¹¹/cm³ or less, or 1×10¹¹/cm³ or less, preferably less than1×10¹⁰/cm³, and 1×10⁻⁹/cm³ or more.

Oxide semiconductor films each having a low impurity concentration and alow density of defect states can be used for the multilayer film 107 andthe oxide semiconductor film 108, in which case the transistors can havemore excellent electrical characteristics. Here, the state in whichimpurity concentration is low and density of defect states is low (theamount of oxygen vacancy is small) is referred to as “highly purifiedintrinsic” or “substantially highly purified intrinsic”. A highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor has few carrier generation sources, and thus has a lowcarrier density in some cases. Thus, a transistor including the oxidesemiconductor film in which a channel region is formed is likely to havepositive threshold voltage (normally-off characteristics). A highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor film has a low density of defect states and accordinglyhas low density of trap states in some cases. Furthermore, a highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor film has an extremely low off-state current; the off-statecurrent can be less than or equal to the measurement limit of asemiconductor parameter analyzer, i.e., less than or equal to 1×10⁻¹³ A,at a voltage (drain voltage) between a source electrode and a drainelectrode of from 1 V to 10 V. Thus, the transistor whose channel regionis formed in the oxide semiconductor film has a small variation inelectrical characteristics and high reliability in some cases.

The oxide semiconductor films 105 and 106 included in the multilayerfilm 107 and the oxide semiconductor film 108 may each have anon-single-crystal structure, for example. The non-single-crystalstructure includes a CAAC-OS which is described later, a polycrystallinestructure, a microcrystalline structure which is described later, or anamorphous structure, for example. Among the non-single crystalstructure, the amorphous structure has the highest density of defectstates, whereas CAAC-OS has the lowest density of defect states.

Note that the oxide semiconductor films 105 and 106 included in themultilayer film 107 and the oxide semiconductor film 108 may be mixedfilms including two or more of the following: a region having anamorphous structure, a region having a microcrystalline structure, aregion having a polycrystalline structure, a CAAC-OS region, and aregion having a single-crystal structure. The mixed film has asingle-layer structure including, for example, two or more of a regionhaving an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure in some cases.Furthermore, the mixed film has a stacked-layer structure of two or moreof a region having an amorphous structure, a region having amicrocrystalline structure, a region having a polycrystalline structure,a CAAC-OS region, and a region having a single-crystal structure in somecases.

Note that in the multilayer film 107, the channel region 107 a and thelow-resistance regions 107 b and 107 c might differ in crystallinity. Inthe oxide semiconductor film 108, the channel region 108 a and thelow-resistance regions 108 b and 108 c might differ in crystallinity.These cases are due to damage to the low-resistance regions 107 b, 107c, 108 b and 108 c, which lowers their crystallinity, when the impurityelement is added to the low-resistance regions 107 b, 107 c, 108 b, and108 c.

The insulating films 116 and 117 can be formed with a single layer orstacked layers using an oxide insulating film or a nitride insulatingfilm. Note that at least regions of the insulating films 116 and 117which are in contact with the multilayer film 107 and the oxidesemiconductor film 108, respectively, are preferably formed using anoxide insulating film, in order to improve characteristics of theinterface with the multilayer film 107 and the oxide semiconductor film108. The insulating films 116 and 117 may be formed with a single layeror stacked layers using one or more of silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, aluminum oxide,hafnium oxide, gallium oxide, a Ga—Zn oxide, and the like.

Furthermore, it is possible to prevent outward diffusion of oxygen fromthe multilayer film 107 and the oxide semiconductor film 108 and entryof hydrogen, water, or the like into the multilayer film 107 and theoxide semiconductor film 108 from the outside by providing an insulatingfilm having a blocking effect against oxygen, hydrogen, water, and thelike as each of the insulating films 116 and 117. As the insulating filmhaving a blocking effect against oxygen, hydrogen, water, and the like,an aluminum oxide film, an aluminum oxynitride film, a gallium oxidefilm, a gallium oxynitride film, an yttrium oxide film, an yttriumoxynitride film, a hafnium oxide film, and a hafnium oxynitride film canbe given as examples.

The insulating films 116 and 117 may be formed using a high-k materialsuch as hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogenis added (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen isadded (HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so thatgate leakage current of the transistors can be reduced.

When the insulating films 116 and 117 are formed using an oxideinsulating film from which oxygen is released by heating, oxygencontained in the insulating films 116 and 117 can be moved to themultilayer film 107 and the oxide semiconductor film 108, respectively,by heat treatment.

The thickness of each of the insulating films 116 and 117 can be greaterthan or equal to 5 nm and less than or equal to 400 nm, greater than orequal to 5 nm and less than or equal to 300 nm, or greater than or equalto 10 nm and less than or equal to 250 nm.

The conductive films 119 and 120 can be formed using a metal elementselected from aluminum, chromium, copper, tantalum, titanium,molybdenum, nickel, iron, cobalt, and tungsten; an alloy containing anyof these metal elements as a component; an alloy containing any of thesemetal elements in combination; or the like. Furthermore, one or moremetal elements selected from manganese and zirconium may be used.Furthermore, the conductive films 119 and 120 may have a single-layerstructure or a stacked-layer structure including two or more layers. Forexample, any of the following can be used: a single-layer structure ofan aluminum film containing silicon; a single-layer structure of acopper film containing manganese; two-layer structure in which atitanium film is stacked over an aluminum film; a two-layer structure inwhich a titanium film is stacked over a titanium nitride film; atwo-layer structure in which a tungsten film is stacked over a titaniumnitride film; a two-layer structure in which a tungsten film is stackedover a tantalum nitride film or a tungsten nitride film; a two-layerstructure in which a copper film is stacked over a copper filmcontaining manganese; a three-layer structure in which a titanium film,an aluminum film, and a titanium film are stacked in this order; athree-layer structure in which a copper film containing manganese, acopper film, and a copper film containing manganese are stacked in thisorder; and the like. Furthermore, an alloy film or a nitride film whichcontains aluminum and one or more elements selected from titanium,tantalum, tungsten, molybdenum, chromium, neodymium, and scandium may beused.

Alternatively, the conductive films 119 and 120 can be formed using alight-transmitting conductive material such as indium tin oxide, indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxideincluding silicon oxide. It is also possible to have a stacked-layerstructure formed using the above light-transmitting conductive materialand the above metal element.

The thickness of each of the conductive films 119 and 120 can be greaterthan or equal to 30 μm and less than or equal to 500 nm, or greater thanor equal to 100 nm and less than or equal to 400 nm.

The conductive films 134, 135, 136, and 137 function as sourceelectrodes and drain electrodes. The conductive films 134, 135, 136, and137 can be formed using any of the materials and structures for theconductive films 119 and 120, as appropriate.

The insulating film 127 can be formed with a single layer or stackedlayers using an oxide insulating film or a nitride insulating film. Whenthe insulating film 127 is formed using an oxide insulating film fromwhich oxygen is released by heating, oxygen contained in the insulatingfilm 127 can be moved to the multilayer film 107 and the oxidesemiconductor film 108 by heat treatment.

The insulating film 127 may be formed with a single layer or stackedlayers using one or more of silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, aluminum oxide, hafnium oxide, galliumoxide, a Ga—Zn oxide, and the like.

The thickness of the insulating film 127 can be greater than or equal to30 nm and less than or equal to 500 nm, or greater than or equal to 100nm and less than or equal to 400 nm.

<Structure 2 of Semiconductor Device>

Next, another structure of the semiconductor device is described withreference to FIGS. 3A and 3B. Here, in a transistor 100 i formed in adriver circuit portion and a transistor 100 j formed in a pixel portion,the conductive films 119 and 120 functioning as gate electrodes eachhave a stacked-layer structure. FIG. 3A shows cross-sectional views ofthe transistors 100 i and 100 j in the channel length direction, andFIG. 3B shows cross-sectional views of the transistors 100 i and 100 jin the channel width direction.

The conductive film 119 includes a conductive film 119 a in contact withthe insulating film 116 and a conductive film 119 b in contact with theconductive film 119 a. The end portion of the conductive film 119 a ispositioned on an outer side than the end portion of the conductive film119 b. In other words, the conductive film 119 a has such a shape thatthe end portion extends beyond the end portion of the conductive film119 b.

The end portion of the insulating film 116 is positioned on an outerside than the end portion of the conductive film 119 a. In other words,the insulating film 116 has such a shape that the end portion extendsbeyond the end portion of the conductive film 119 a. Furthermore, a sidesurface of the insulating film 116 may be curved.

The conductive film 120 includes a conductive film 120 a in contact withthe insulating film 117 and a conductive film 120 b in contact with theconductive film 120 a. The end portion of the conductive film 120 a ispositioned on an outer side than the end portion of the conductive film120 b. In other words, the conductive film 120 a has such a shape thatthe end portion extends beyond the end portion of the conductive film120 b.

The end portion of the insulating film 117 is positioned on an outerside than the end portion of the conductive film 120 a. In other words,the insulating film 117 has such a shape that the end portion extendsbeyond the end portion of the conductive film 120 a. Furthermore, a sidesurface of the insulating film 117 may be curved.

The conductive films 119 a and 120 a can be formed using titanium,tantalum, molybdenum, tungsten, an alloy of any of these, titaniumnitride, tantalum nitride, molybdenum nitride, tungsten nitride, or thelike. Alternatively, the conductive films 119 a and 120 a can be formedusing a Cu—X alloy (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) or the like.

The conductive films 119 b and 120 b are formed using a low-resistancematerial. The conductive films 119 b and 120 b can be formed usingcopper, aluminum, gold, silver, tungsten, or the like, an alloycontaining any of these, a compound containing any of these as a maincomponent, or the like.

In the case where the Cu—X alloy (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, orTi) is used for the conductive films 119 a and 120 a, in a region ofeach of the conductive films 119 a and 120 a which is in contact with aninsulating film, a covering film is formed by heat treatment in somecases. The covering film includes a compound containing X. Examples ofthe compound containing X include an oxide of X and a nitride of X. Whenthe covering film is formed on surfaces of the conductive films 119 aand 120 a, the covering film functions as a blocking film, and Cu in theCu—X alloy film can be prevented from entering the oxide semiconductorfilms.

Note that when the concentrations of copper in the channel regions inthe multilayer film 107 and the oxide semiconductor film 108 are lowerthan or equal to 1×10¹⁸ atoms/cm³, electron trap state density at theinterface between the multilayer film 107 and the insulating film 116functioning as a gate insulating film and the interface between theoxide semiconductor film 108 and the insulating film 117 functioning asa gate insulating film can be reduced. As a result, transistors eachhaving an excellent subthreshold swing value (S value) can bemanufactured.

When the conductive films 119 and 120 and the insulating films 116 and117 having the shapes shown in FIGS. 3A and 3B are provided in thetransistors 100 i and 100 j, the electric field of the drain region ofeach of the transistors can be relaxed. Thus, deterioration of thetransistor due to the electric field of the drain region, such as ashift of the threshold voltage of the transistor, can be inhibited.

<Structure 3 of Semiconductor Device>

Next, another structure of the semiconductor device is described withreference to FIGS. 7A and 7B and FIGS. 8A and 8B. Here, a transistor 100k formed in a driver circuit portion has a dual-gate structure. FIG. 7Ais the top view of the transistor 100 k, and FIG. 7B is the top view ofthe transistor 100 z. FIG. 8A shows cross-sectional views along thedashed-dotted line A-B in FIG. 7A and the dashed-dotted line C-D in FIG.7B. FIG. 8B shows cross-sectional views along the dashed-dotted line G-Hin FIG. 7A and the dashed-dotted line I-J in FIG. 7B.

The transistor 100 k shown in FIGS. 8A and SB includes a conductive film102 over the substrate 101, the insulating film 104 over the substrate101 and the conductive film 102, the multilayer film 107 over theinsulating film 104, the insulating film 116 in contact with themultilayer film 107, and the conductive film 119 overlapping with themultilayer film 107 with the insulating film 116 provided therebetween.The structure of the multilayer film 107 is similar to that of themultilayer film 107 described in <Structure 1 of semiconductor device>and therefore is not described in detail here.

The conductive films 102 and 119 function as gate electrodes. That is,the transistor 100 k is a transistor having a dual-gate structure. Theinsulating films 104 and 116 function as gate insulating films.

Note that, although not shown, the conductive film 102 may overlap withan entire region of the multilayer film 107.

A nitride insulating film 161 is preferably provided over the substrate101. Examples of the nitride insulating film 161 include a siliconnitride film and an aluminum nitride film. Covering the conductive film102 with the nitride insulating film 161 and the insulating film 104 amakes it possible to prevent diffusion of metal elements contained inthe conductive film 102.

In the transistor 100 k, different potentials are supplied to theconductive film 102 and the conductive film 119 which are not connectedto each other; thus, the threshold voltage of the transistor 100 k canbe controlled. Alternatively, as shown in FIG. 8B, by supplying the samepotential to the conductive film 102 and the conductive film 119 whichare connected to each other, variations in the initial characteristicscan be reduced, and degradation of the transistor due to the −GBT(negative gate bias-temperature) stress test and a change in the risingvoltage of the on-state current at different drain voltages can besuppressed. Furthermore, when the conductive film 102 and the conductivefilm 119 are connected to each other as shown in FIG. 8B, electricfields of the conductive films 102 and 119 affect a top surface and aside surface of the multilayer film 107, so that carriers flow in theentire multilayer film 107. In other words, a region where carriers flowbecomes larger in the film thickness direction, so that the amount ofcarrier movement is increased. As a result, the on-state current andfield-effect mobility of the transistor 100 k are increased. When thechannel length of the transistor is less than 2.5 μm, preferably greaterthan or equal to 1.45 μm and less than or equal to 2.2 μm, the on-statecurrent can be further increased and the field-effect mobility can beincreased. Owing to the high on-state current, the transistor 100 k canhave a small plane area. Consequently, a display device with a narrowbezel in which the area occupied by a driver circuit portion is smallcan be manufactured.

The transistor 100 z can have a structure similar to that of thetransistor 100 h described in <Structure 1 of semiconductor device> andtherefore is not described in detail here.

In the display device described in this embodiment, the transistor inthe driver circuit portion and the transistor in the pixel portion havedifferent structures. The transistor included in the driver circuitportion has a dual-gate structure. That is, the field-effect mobility ofthe transistor included in the driver circuit portion is higher thanthat of the transistor included in the pixel portion. Consequently, adriver circuit portion capable of high-speed operation can be formed.Furthermore, when a driver circuit portion is formed using a transistorcapable of high-speed operation and a pixel portion is formed using atransistor with small parasitic capacitance and small parasiticresistance, a high-resolution display device capable of double-framerate driving can be manufactured.

<Structure 4 of Semiconductor Device>

Next, another structure of the semiconductor device is described withreference to FIGS. 9A and 9B, Here, in a transistor 100 m formed in adriver circuit portion and a transistor 100 n formed in a pixel portion,the conductive films 119 and 120 functioning as gate electrodes eachhave a stacked-layer structure. FIG. 9A shows cross-sectional views ofthe transistors 100 m and 100 n in the channel length direction, andFIG. 9B shows cross-sectional views of the transistors 100 m and 100 nin the channel width direction.

The conductive films 119 and 120 have stacked-layer structures similarto those of the conductive films 119 and 120 described in <Structure 2of semiconductor device> in this embodiment.

The insulating films 116 and 117 have structures similar to those of theinsulating films 116 and 117 described in <Structure 2 of semiconductordevice> in this embodiment.

When the conductive films 119 and 120 and the insulating films 116 and117 having the shapes shown in FIGS. 9A and 9B are provided in thetransistors 100 m and 100 n, the electric field of the drain region ofeach of the transistors can be relaxed. Thus, deterioration of thetransistor due to the electric field of the drain region, such as ashift of the threshold voltage of the transistor, can be inhibited.

<Structure 5 of Semiconductor Device>

Next, another structure of the semiconductor device is described withreference to FIGS. 10A and 10B. Here, transistors 111 w and 111 x formedin a driver circuit portion and a transistor 111 y formed in a pixelportion each include a multilayer film. FIG. 10A shows cross-sectionalviews of the transistors 111 w and 100 h in the channel lengthdirection, and FIG. 10B shows cross-sectional views of the transistors111 x and 111 y in the channel length direction.

The multilayer film 107 included in the transistor 111 w in FIG. 10Aincludes the channel region 107 a and the low-resistance regions 107 band 107 c. The channel region 107 a includes a channel region 142 a incontact with the insulating film 104, the channel region 105 a incontact with the channel region 142 a, and the channel region 106 a incontact with the channel region 105 a. The low-resistance region 107 bincludes a low-resistance region 142 b in contact with the insulatingfilm 104, the low-resistance region 105 b in contact with thelow-resistance region 142 b, and the low-resistance region 106 b incontact with the low-resistance region 105 b. The low-resistance region107 c includes a low-resistance region 142 c in contact with theinsulating film 104, the low-resistance region 105 c in contact with thelow-resistance region 142 c, and the low-resistance region 106 c incontact with the low-resistance region 105 c. Note that although notshown in FIGS. 10A and 10B, an oxide semiconductor film including thechannel region 142 a, the low-resistance region 142 b, and thelow-resistance region 142 c is referred to as an oxide semiconductorfilm 142. That is, in the multilayer film 107, the oxide semiconductorfilm 142, the oxide semiconductor film 105, and the oxide semiconductorfilm 106 are stacked in this order.

The multilayer film 107 included in the transistor 111 x in FIG. 10Bincludes the channel region 107 a and the low-resistance regions 107 band 107 c. The channel region 107 a includes the channel region 142 a incontact with the insulating film 104, the channel region 105 a incontact with the channel region 142 a, the channel region 106 a incontact with the channel region 105 a, and a channel region 143 a incontact with the channel region 106 a. The low-resistance region 107 bincludes the low-resistance region 142 b in contact with the insulatingfilm 104, the low-resistance region 105 b in contact with thelow-resistance region 142 b, the low-resistance region 106 b in contactwith the low-resistance region 105 b, and a low-resistance region 143 bin contact with the low-resistance region 106 b. The low-resistanceregion 107 c includes the low-resistance region 142 c in contact withthe insulating film 104, the low-resistance region 105 c in contact withthe low-resistance region 142 c, the low-resistance region 106 c incontact with the low-resistance region 105 c, and a low-resistanceregion 143 c in contact with the low-resistance region 106 c. Note thatalthough not shown in FIGS. 10A and 10B, an oxide semiconductor filmincluding the channel region 143 a, the low-resistance region 143 b, andthe low-resistance region 143 c is referred to as an oxide semiconductorfilm 143. That is, in the multilayer film 107, the oxide semiconductorfilm 142, the oxide semiconductor film 105, the oxide semiconductor film106, and the oxide semiconductor film 143 are stacked in this order.

A multilayer film 110 included in the transistor 111 y in FIG. 10Bincludes a channel region 110 a and low-resistance regions 110 b and 110c. The channel region 110 a includes the channel region 108 a in contactwith the insulating film 104 and a channel region 144 a in contact withthe channel region 108 a. The low-resistance region 110 b includes thelow-resistance region 108 b in contact with the insulating film 104 anda low-resistance region 144 b in contact with the low-resistance region108 b. The low-resistance region 110 c includes the low-resistanceregion 108 c in contact with the insulating film 104 and alow-resistance region 144 c in contact with the low-resistance region108 c. Note that although not shown in FIGS. 10A and 10B, an oxidesemiconductor film including the channel region 144 a, thelow-resistance region 144 b, and the low-resistance region 144 c isreferred to as an oxide semiconductor film 144. That is, in themultilayer film 110, the oxide semiconductor film 108 and the oxidesemiconductor film 144 are stacked in this order.

The oxide semiconductor films 142, 143, and 144 each preferably have alarger energy gap, a lower electron affinity, and a higher insulatingproperty than the oxide semiconductor films 105, 106, and 108. Inaddition, the oxide semiconductor films 142, 143, and 144 eachpreferably have a lower indium content than the oxide semiconductorfilms 105, 106, and 108. The oxide semiconductor films 142, 143, and 144preferably have a function of blocking external impurity. In such anoxide semiconductor film, the proportion of In atoms is lower than thatof M (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce, Nd, or HO atoms. In the casewhere the oxide semiconductor films 142, 143, and 144 contain an In-M-Znoxide (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce, Nd, or HO, and a targethaving the atomic ratio of the metal elements of In:M:Zn=x₃:y₃:z₃ isused for forming the oxide semiconductor films 142, 143, and 144, x₃/y₃is preferably greater than or equal to ⅙ and less than 1; z₃/y₃ ispreferably greater than or equal to ⅓ and less than or equal to 6,further preferably greater than or equal to 1 and less than or equal to6. Note that when z₃/y₃ is greater than or equal to 1 and less than orequal to 6, CAAC-OS films are easily formed as the oxide semiconductorfilms 142, 143, and 144. Typical examples of the atomic ratio of themetal elements of the target are In:M:Zn=1:3:2, In:M:Zn=1:3:4,In:M:Zn=1:3:6, In:M:Zn=1:3:8, In:M:Zn=1:4:4, In:M:Zn=1:4:5,In:M:Zn=1:4:6, In:M:Zn=1:4:7, In:M:Zn=1:4:8, In:M:Zn=1:5:5,In:M:Zn=1:5:6, In:M:Zn=1:5:7, In:M:Zn=1:5:8, and In:M:Zn=1:6:8.

In the transistor 111 w shown in FIG. 10A, the oxide semiconductor film142 has a larger energy gap and a lower electron affinity than the oxidesemiconductor film 105; therefore, a channel is formed in the oxidesemiconductor film 105. That is, a buried channel structure is obtained.The oxide semiconductor film 106 and the oxide semiconductor film 142contain one or more kinds of metal elements that form the oxidesemiconductor film 105. Therefore, an interface state is not easilyformed at the interface between the oxide semiconductor film 105 and theoxide semiconductor film 106 and at the interface between the oxidesemiconductor film 105 and the oxide semiconductor film 142. Thus,providing the oxide semiconductor film 106 and the oxide semiconductorfilm 142 makes it possible to reduce variations or changes in electricalcharacteristics of the transistor, such as threshold voltage.

Similarly, in the transistor 111 x shown in FIG. 10B, the oxidesemiconductor films 142 and 143 each have a larger energy gap and alower electron affinity than the oxide semiconductor films 105 and 106;therefore, a channel is formed in the oxide semiconductor film 105. Thatis, a buried channel structure is obtained. The multilayer film 107includes the oxide semiconductor films 142 and 143. Therefore, aninterface state is not easily formed at the interface between the oxidesemiconductor film 142 and the oxide semiconductor film 105, at theinterface between the oxide semiconductor film 105 and the oxidesemiconductor film 106, and at the interface between the oxidesemiconductor film 106 and the oxide semiconductor film 143.Consequently, variations or changes in electrical characteristics of thetransistor, such as threshold voltage, can be reduced.

Similarly, in the transistor 111 y shown in FIG. 10B, the multilayerfilm 110 includes the oxide semiconductor film 144; therefore, aninterface state is not easily formed at the interface between the oxidesemiconductor film 144 and the oxide semiconductor film 108.Consequently, variations or changes in electrical characteristics of thetransistor, such as threshold voltage, can be reduced.

<Band Structure>

Next, band structures along given cross sections of the transistor 100 kin FIGS. 8A and 8B, which is a typical example of a transistor of thisembodiment, are described.

A band structure in the O—P cross section including the channel regionsof the transistor 100 k in FIG. 8A is illustrated in FIG. 28A. Thechannel region 106 a has a slightly larger energy gap than the channelregion 105 a. The insulating film 104 a, the insulating film 104 b, andthe insulating film 116 each have a sufficiently larger energy gap thanthe channel region 106 a and the channel region 105 a. Furthermore, theFermi levels (denoted by Ef) of the channel region 106 a, the channelregion 105 a, the insulating film 104 a, the insulating film 104 b, andthe insulating film 116 are assumed to be equal to the intrinsic Fermilevels thereof (denoted by Ei). Furthermore, work functions of theconductive film 102 and the conductive film 119 are assumed to be equalto the Fenui levels.

When a gate voltage is set to be higher than or equal to the thresholdvoltage of the transistor, an electron flows preferentially in thechannel region 105 a owing to the difference between the energies of theconduction band minimums of the channel region 106 a and the channelregion 105 a. That is, it is probable that an electron is embedded inthe channel region 105 a. Note that the energy at the conduction bandminimum is denoted by Ec, and the energy at the valence band maximum isdenoted by Ev.

Accordingly, in the transistor of one embodiment of the presentinvention, the embedment of an electron reduces the influence ofinterface scattering. Therefore, the channel resistance of thetransistor of one embodiment of the present invention is low.

Next, FIG. 28B shows a band structure in the Q-R cross section includingthe source region or the drain region of the transistor 100 k in FIG.8A. Note that the low-resistance regions 105 b, 105 c, 106 b, and 106 care assumed to be in a degenerate state. That is, in each of thelow-resistance regions 105 b, 105 c, 106 b, and 106 c, the Fermi levelEf is assumed to be approximately the same as the energy Ec of theconduction band minimum. Furthermore, in the low-resistance region 105b, the energy of the conduction band minimum is assumed to beapproximately the same as the Fermi level of the channel region 105 a.Furthermore, in the low-resistance region 106 b, the energy of theconduction band minimum is assumed to be approximately the same as theFermi level of the channel region 106 a. The same applies to thelow-resistance region 105 c and the low-resistance region 106 c.

At this time, an ohmic contact is made between the conductive film 134and the low-resistance region 106 b because an energy barriertherebetween is sufficiently low. Furthermore, an ohmic contact is madebetween the low-resistance region 106 b and the low-resistance region105 b. Similarly, an ohmic contact is made between the conductive film135 and the low-resistance region 106 c because an energy barriertherebetween is sufficiently low. Furthermore, an ohmic contact is madebetween the low-resistance region 106 c and the low-resistance region105 c. Therefore, electron transfer is conducted smoothly between theconductive films 134 and 135 and the channel regions 106 a and 105 a.

As described above, the transistor of one embodiment of the presentinvention is a transistor in which the channel resistance is low andelectron transfer between the channel region and the source and thedrain electrodes is conducted smoothly. That is, the transistor hasexcellent switching characteristics.

<Method 1 for Manufacturing Semiconductor Device>

Next, a method for manufacturing the transistors 100 g and 100 hillustrated in FIGS. 1A and 1B and FIGS. 2A and 2B will be describedwith reference to FIGS. 4A to 4C, FIGS. 5A to 5C, and FIGS. 6A and 6B.

The films included in the transistors 100 g and 100 h (i.e., theinsulating film, the oxide semiconductor film, the conductive film, andthe like) can be formed by any of a sputtering method, a chemical vapordeposition (CVD) method, a vacuum evaporation method, and a pulsed laserdeposition (PLD) method. Alternatively, a coating method or a printingmethod can be used. Although the sputtering method and a plasma-enhancedchemical vapor deposition (PECVD) method are typical examples of thefilm formation method, a thermal CVD method may be used. As the thermalCVD method, a metal organic chemical vapor deposition (MOCVD) method oran atomic layer deposition (ALD) method may be used, for example.

Deposition by a thermal CVD method may be performed in such a mannerthat the pressure in a chamber is set to an atmospheric pressure or areduced pressure, and a source gas and an oxidizer are supplied to thechamber at a time and react with each other in the vicinity of thesubstrate or over the substrate. Thus, no plasma is generated in thedeposition; therefore, the thermal CVD method has an advantage that nodefect due to plasma damage is caused.

Deposition by the ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). For example, a first source gas is introduced, aninert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after the introduction of the first source gas so thatthe source gases are not mixed, and then a second source gas isintroduced. Note that in the case where the first source gas and theinert gas are introduced at a time, the inert gas serves as a carriergas, and the inert gas may also be introduced at the same time as theintroduction of the second source gas. Alternatively, the first sourcegas may be exhausted by vacuum evacuation instead of the introduction ofthe inert gas, and then the second source gas may be introduced. Thefirst source gas is adsorbed on the surface of the substrate to form afirst single-atomic layer; then the second source gas is introduced toreact with the first single-atomic layer; as a result, a secondsingle-atomic layer is stacked over the first single-atomic layer, sothat a thin film is formed.

The sequence of the gas introduction is repeated plural times until adesired thickness is obtained, whereby a thin film with excellent stepcoverage can be formed. The thickness of the thin film can be adjustedby the number of repetition times of the sequence of the gasintroduction; therefore, an ALD method makes it possible to accuratelyadjust a thickness and thus is suitable for manufacturing a minute FET.

As illustrated in FIG. 4A, the insulating film 104 is formed over thesubstrate 101. Next, the oxide semiconductor film 105 is formed over theinsulating film 104 in the driver circuit portion.

The insulating film 104 can be formed by a sputtering method, a CVDmethod, an evaporation method, a pulsed laser deposition (PLD) method, aprinting method, a coating method, or the like, as appropriate. Theinsulating film 104 can be formed in the following manner: an insulatingfilm is formed over the substrate 101, and then oxygen is added to theinsulating film. Examples of the oxygen that is added to the insulatingfilm include an oxygen radical, an oxygen atom, an oxygen atomic ion, anoxygen molecular ion, and the like. As a method for adding the oxygen,an ion doping method, an ion implantation method, plasma treatment, orthe like can be given. Alternatively, after a film which suppressesrelease of oxygen is formed over the insulating film, oxygen may beadded to the insulating film through the film.

As the insulating film 104, a silicon oxide film or a silicon oxynitridefilm from which oxygen can be released by heat treatment can be formedunder the following conditions: the substrate placed in a treatmentchamber of the plasma CVD apparatus that is vacuum-evacuated is held ata temperature higher than or equal to 180° C. and lower than or equal to280° C., or higher than or equal to 200° C. and lower than or equal to240° C., the pressure is greater than or equal to 100 Pa and less thanor equal to 250 Pa, or greater than or equal to 100 Pa and less than orequal to 200 Pa with introduction of a source gas into the treatmentchamber, and a high-frequency power of greater than or equal to 0.17W/cm² and less than or equal to 0.5 W/cm², or greater than or equal to0.25 W/cm² and less than or equal to 0.35 W/cm² is supplied to anelectrode provided in the treatment chamber.

Here, the insulating film 104 a and the insulating film 104 b may bestacked to form the insulating film 104. For example, a 100-nm-thicksilicon nitride film is formed by a plasma CVD method as the insulatingfilm 104 a, and a 300-nm-thick silicon oxynitride film is formed by aplasma CVD method as the insulating film 104 b.

A formation method of the oxide semiconductor film 105 is describedbelow. An oxide semiconductor film is formed over the insulating film104 by a sputtering method, a coating method, a pulsed laser depositionmethod, a laser ablation method, a thermal CVD method, or the like.Then, after a mask is formed over the oxide semiconductor film by alithography process, the oxide semiconductor film is partly etched usingthe mask. Thus, the oxide semiconductor film 105 can be formed asillustrated in FIG. 4A. After that, the mask is removed. Note that heattreatment may be performed after the oxide semiconductor film 105 isformed by etching part of the oxide semiconductor film.

Alternatively, by using a printing method for forming the oxidesemiconductor film 105, the oxide semiconductor film 105 subjected toelement isolation can be formed directly.

As a power supply device for generating plasma in the case of formingthe oxide semiconductor film by a sputtering method, an RF power supplydevice, an AC power supply device, a DC power supply device, or the likecan be used as appropriate. Note that a CAAC-OS film can be formed usingan AC power supply device or a DC power supply device. In forming theoxide semiconductor film, a sputtering method using an AC power supplydevice or a DC power supply device is preferable to a sputtering methodusing an RF power supply device because the oxide semiconductor film canbe uniform in film thickness, film composition, or crystallinity.

As a sputtering gas, a rare gas (typically argon), an oxygen gas, or amixed gas of a rare gas and oxygen is used as appropriate. In the caseof using the mixed gas of a rare gas and oxygen, the proportion ofoxygen to a rare gas is preferably increased.

The oxide semiconductor film 105 is formed using an In-M-Zn oxide (M isMg, Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf) target. When the atomic ratioof the metal elements of the target is In:M:Zn=x₁:y₁:z₁, x₁/y₁ isgreater than 1 and less than or equal to 6.

For example, in the case where the oxide semiconductor film is formed bya sputtering method at a substrate temperature higher than or equal to150° C. and lower than or equal to 750° C., higher than or equal to 150°C. and lower than or equal to 450° C., or higher than or equal to 200°C. and lower than or equal to 350° C., a CAAC-OS film can be formed. Inthe case where the substrate temperature is higher than or equal to 25°C. and lower than 150° C., a microcrystalline oxide semiconductor filmcan be formed.

For the deposition of the CAAC-OS film to be described later, thefollowing conditions are preferably used.

By suppressing entry of impurities during the deposition, the crystalstate can be prevented from being broken by the impurities. For example,the concentration of impurities (e.g., hydrogen, water, carbon dioxide,or nitrogen) which exist in the deposition chamber may be reduced.Furthermore, the concentration of impurities in a deposition gas may bereduced. Specifically, a deposition gas whose dew point is −80° C. orlower, or −100° C. or lower is used.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is 30 vol. % or higher, or 100 vol. %.

Furthermore, after the oxide semiconductor film is formed, heattreatment may be performed so that the oxide semiconductor film issubjected to dehydrogenation or dehydration. The heat treatment isperformed typically at a temperature higher than or equal to 150° C. andlower than the strain point of the substrate, higher than or equal to250° C. and lower than or equal to 450° C., or higher than or equal to300° C. and lower than or equal to 450° C.

The heat treatment is performed under an inert gas atmosphere containingnitrogen or a rare gas such as helium, neon, argon, xenon, or krypton.Alternatively, the heat treatment may be performed under an inert gasatmosphere first, and then under an oxygen atmosphere. It is preferablethat the above inert gas atmosphere and the above oxygen atmosphere donot contain hydrogen, water, and the like. The treatment time is from 3minutes to 24 hours.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With the use of an RTA apparatus, the heat treatment canbe performed at a temperature of higher than or equal to the strainpoint of the substrate if the heating time is short. Therefore, the heattreatment time can be shortened.

By forming the oxide semiconductor film while it is heated or performingheat treatment after the formation of the oxide semiconductor film, thehydrogen concentration in the oxide semiconductor film which is measuredby SIMS can be 5×10¹⁹ atoms/cm³ or lower, 1×10¹⁹ atoms/cm³ or lower,5×10¹⁸ atoms/cm³ or lower, 1×10¹⁸ atoms/cm³ or lower, 5×10¹⁷ atoms/cm³or lower, or 1×10¹⁶ atoms/cm³ or lower.

For example, in the case where an oxide semiconductor film, e.g., anInGaZnO_(X) (X>0) film is formed using a deposition apparatus employingALD, an In(CH₃)₃ gas and an O₃ gas are sequentially introduced more thanonce to form an InO₂ layer, a Ga(CH₃)₃ gas and an O₃ gas are introducedat a time to form a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas areintroduced at a time to form a ZnO layer. Note that the order of theselayers is not limited to this example. A mixed compound layer such as anInGaO₂ layer, an InZnO₂ layer, a GaInO layer, a ZnInO layer, or a GaZnOlayer may be formed by mixing of these gases. Note that although an H₂Ogas which is obtained by bubbling with an inert gas such as Ar may beused instead of an O₃ gas, it is preferable to use an O₃ gas, which doesnot contain H. Instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used.Instead of a Ga(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used. Furthermore,instead of a Zn(CH₃)₂ gas, a Zn(C₂H₅)₂ gas may be used.

Here, a 35-nm-thick oxide semiconductor film is formed by a sputteringmethod, Next, a mask is formed over the oxide semiconductor film, andpart of the oxide semiconductor film is selectively etched. In thismanner, the oxide semiconductor film 105 is formed. As the oxidesemiconductor film 105, an In—Ga—Zn oxide film (In:Ga:Zn=3:1:2) isformed.

Next, as shown in FIG. 4B, the oxide semiconductor film 106 is formedover the oxide semiconductor film 105 in the driver circuit portion, andthe oxide semiconductor film 108 is formed in the pixel portion. Thus,the multilayer film 107 in which the oxide semiconductor film 105 andthe oxide semiconductor film 106 are stacked in this order is formed.

Note that in this step, the oxide semiconductor film 106 is formed tocover a top surface and a side surface of the oxide semiconductor film105, to prevent the oxide semiconductor film 105 from being etched in alater step of forming conductive films functioning as a source electrodeand a drain electrode. This is preferable because variations in thelength of the oxide semiconductor film 105 in the channel widthdirection of the transistor can be reduced.

Here, a 20-nm-thick oxide semiconductor film is formed by a sputteringmethod. Next, a mask is formed over the oxide semiconductor film, andpart of the oxide semiconductor film is selectively etched. In thismanner, the oxide semiconductor films 106 and 108 are formed. As theoxide semiconductor films 106 and 108, In—Ga—Zn oxide films(In:Ga:Zn=1:1:1.2) are formed.

Next, oxygen contained in the insulating film 104 is moved to the oxidesemiconductor films by heat treatment. Note that the heat treatment maybe performed at a time that is after the formation of the oxidesemiconductor film to be the oxide semiconductor films 106 and 108 andbefore the etching of the oxide semiconductor film for forming the oxidesemiconductor films 106 and 108.

When the heat treatment is performed at a temperature higher than 350°C. and lower than or equal to 650° C., or higher than or equal to 450°C. and lower than or equal to 600° C., it is possible to obtain an oxidesemiconductor film whose proportion of CAAC, which is described later,is greater than or equal to 60% and less than 100%, greater than orequal to 80% and less than 100%, greater than or equal to 90% and lessthan 100%, or greater than or equal to 95% and less than or equal to98%. Furthermore, it is possible to obtain an oxide semiconductor filmhaving a low content of hydrogen, water, and the like. That is, an oxidesemiconductor film with a low impurity concentration and a low densityof defect states can be formed.

Next, as shown in FIG. 4C, an insulating film 115 is formed over theinsulating film 104, the multilayer film 107, and the oxidesemiconductor film 108. Then, the conductive films 119 and 120 areformed over the insulating film 115.

In the case where the conductive films 119 and 120 are forming using,for example, a low-resistance material, entry of the low-resistancematerial into the oxide semiconductor films leads to poor electricalcharacteristics of the transistors. In this embodiment, the insulatingfilm 115 is formed before the conductive films 119 and 120 are formed;thus, the channel region in each of the oxide semiconductor films 105and 108 is not in contact with the conductive films 119 and 120.Therefore, the variation in the electrical characteristics, typicallythreshold voltage, of the transistors can be suppressed.

As the insulating film 115, a silicon oxide film or a silicon oxynitridefilm can be formed by a CVD method. In this case, a deposition gascontaining silicon and an oxidizing gas are preferably used as a sourcegas. Typical examples of the deposition gas containing silicon includesilane, disilane, trisilane, and silane fluoride. Examples of theoxidizing gas include oxygen, ozone, dinitrogen monoxide, and nitrogendioxide.

A silicon oxynitride film with few defects can be formed as theinsulating film 115 by a CVD method under the conditions where the ratioof an oxidizing gas to a deposition gas is higher than 20 times andlower than 100 times, or higher than or equal to 40 times and lower thanor equal to 80 times and the pressure in a treatment chamber is lowerthan 100 Pa, or lower than or equal to 50 Pa.

A silicon oxide film or a silicon oxynitride film which is dense can beformed as the insulating film 115 under the following conditions: thesubstrate placed in a treatment chamber of a plasma CVD apparatus thatis vacuum-evacuated is held at a temperature higher than or equal to280° C. and lower than or equal to 400° C., the pressure in thetreatment chamber is greater than or equal to 20 Pa and less than orequal to 250 Pa, preferably greater than or equal to 100 Pa and lessthan or equal to 250 Pa with introduction of a source gas into thetreatment chamber, and a high-frequency power is supplied to anelectrode provided in the treatment chamber.

The insulating film 115 can be formed by a plasma CVD method using amicrowave. The microwave refers to a wave in the frequency range of 300MHz to 300 GHz. In the case of a microwave, electron temperature is lowand electron energy is low. Furthermore, in supplied power, theproportion of power used for acceleration of electrons is low, andtherefore, power can be used for dissociation and ionization of moremolecules. Thus, plasma with high density (high-density plasma) can beexcited. Therefore, a deposition surface and a deposit are less damagedby plasma, and the insulating film 115 with few defects can be formed.

Alternatively, the insulating film 115 can be formed by a CVD methodusing an organosilane gas. As the organosilane gas, any of the followingsilicon-containing compound can be used: tetraethyl orthosilicate (TEOS)(chemical formula: Si(OC₂H₅)₄); tetramethylsilane (TMS) (chemicalformula: Si(CH₃)₄); tetramethylcyclotetrasiloxane (TMCTS);octamethylcyclotetrasiloxane (OMCTS); hexamethyldisilazane (HMDS);triethoxysilane (SiH(OC₂H₅)₃); trisdimethylaminosilane (SiH(N(CH₃)₂)₃);or the like. The insulating film 115 having high coverage can be formedby a CVD method using an organosilane gas.

In the case where a gallium oxide film is formed as the insulating film115, an MOCVD method can be used.

In the case where a hafnium oxide film is formed as the insulating film115 by a thermal CVD method such as an MOCVD method or an ALD method,two kinds of gases, i.e., ozone (O₃) as an oxidizer and a source gaswhich is obtained by vaporizing a liquid containing a solvent and ahafnium precursor compound (a hafnium alkoxide solution, which istypified by tetrakis(dimethylamide)hafnium (TDMAH)), are used. Note thatthe chemical formula of tetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄.Examples of another material liquid includetetrakis(ethylmethylamide)hafnium.

In the case where an aluminum oxide film is formed as the insulatingfilm 115 by a thermal CVD method such as an MOCVD method or an ALDmethod, two kinds of gases, i.e., H₂O as an oxidizer and a source gaswhich is obtained by vaporizing a liquid containing a solvent and analuminum precursor compound (e.g., trimethylaluminum (TMA)) are used.Note that the chemical formula of trimethylaluminum is Al(CH₃)₃.Examples of another material liquid include tris(dimethylamide)aluminum,triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate). Note that the ALD methodenables the insulating film 115 to have excellent coverage and smallthickness.

In the case where a silicon oxide film is formed as the insulating film115 by a thermal CVD method such as an MOCVD method or an ALD method,hexachlorodisilane is adsorbed on a deposition surface, chlorinecontained in adsorbate is removed, and radicals of an oxidizing gas(e.g., O₂ or dinitrogen monoxide) are supplied to react with theadsorbate.

Here, a 100-nm-thick silicon oxynitride film is formed by a plasma CVDmethod as the insulating film 115.

The conductive films 119 and 120 are formed as follows: a conductivefilm is formed by a sputtering method, a vacuum evaporation method, apulsed laser deposition (PLD) method, a thermal CVD method, or the like,a mask is formed over the conductive film by a lithography process, andthen etching treatment is performed.

Alternatively, a tungsten film can be formed as the conductive film witha deposition apparatus employing ALD. In that case, a WF₆ gas and a B₂H₆gas are sequentially introduced more than once to form an initialtungsten film, and then a WF₆ gas and an H₂ gas are introduced at atime, so that a tungsten film is formed. Note that an SiH₄ gas may beused instead of a B₂H₆ gas.

Here, masks 122 and 123 are formed over the conductive film by alithography process and then the conductive film is etched, whereby theconductive films 119 and 120 are formed.

Note that the conductive films 119 and 120 may be formed by anelectrolytic plating method, a printing method, an inkjet method, or thelike instead of the above formation method.

Then, as shown in FIG. 5A, the insulating film 115 is etched with themasks 122 and 123 left, so that the insulating films 116 and 117 areformed.

Next, as shown in FIG. 5B, an impurity element 125 is added to themultilayer film 107 and the oxide semiconductor film 108 with the masks122 and 123 left. As a result, the impurity element is added to regionswhich are not covered with the masks 122 and 123 in the multilayer film107 and the oxide semiconductor film 108. Note that by the addition ofthe impurity element 125, an oxygen vacancy is formed in the multilayerfilm 107 and the oxide semiconductor film 108.

Alternatively, after the masks 122 and 123 are removed, a film(typically, a nitride insulating film, an oxide insulating film, or thelike) with a thickness such that the impurity element 125 can be addedto the oxide semiconductor films may be formed and the impurity element125 may be added to the oxide semiconductor films. The thickness suchthat the impurity element 125 can be added to the oxide semiconductorfilms is greater than or equal to 0.1 nm and less than or equal to 50nm, or greater than or equal to 1 nm and less than or equal to 10 nm.

As a method for adding the impurity element 125, an ion doping method,an ion implantation method, plasma treatment, or the like can be given.In the case of plasma treatment, plasma is generated in a gas atmospherecontaining an impurity element to be added and plasma treatment isperformed, whereby the impurity element can be added. A dry etchingapparatus, a plasma CVD apparatus, a high-density plasma CVD apparatus,or the like can be used to generate the plasma. In the case of plasmatreatment, the substrate 101 may be set to a parallel plate electrode onthe cathode side and an RF power may be supplied so that a bias isapplied to the substrate 101 side. As the RF power, for example, powerdensity can be greater than or equal to 0.1 W/cm² and less than or equalto 2 W/cm². Consequently, the amount of impurity elements added to themultilayer film 107 and the oxide semiconductor film 108 can beincreased and more oxygen vacancies can be formed in the multilayer film107 and the oxide semiconductor film 108.

Note that, as a source gas of the impurity element 125, one or more ofB₂H₆, PH₃, CH₄, N₂, NH₃, AlH₃, AlCl₃, SiH₄, Si₂H₆, F₂, HF, H₂, and arare gas can be used. Alternatively, one or more of B₂H₆, PH₃, N₂, NH₃,AlH₃, MCl₃, F₂, HF, and H₂ which are diluted with a rare gas can beused. By adding the impurity element 125 to the multilayer film 107 andthe oxide semiconductor film 108 using one or more of B₂H₆, PH₃, N₂,NH₃, AlH₃, AlCl₃, F₂, HF, and H₂ which are diluted with a rare gas, therare gas and one or more of hydrogen, boron, carbon, nitrogen, fluorine,aluminum, silicon, phosphorus, and chlorine can be added at a time tothe multilayer film 107 and the oxide semiconductor film 108.

Alternatively, after a rare gas is added to the multilayer film 107 andthe oxide semiconductor film 108, one or more of B₂H₆, PH₃, CH₄, N₂,NH₃, AlH₃, AlCl₃, Si₂H₆, F₂, HF, and H₂ may be added to the multilayerfilm 107 and the oxide semiconductor film 108.

Further alternatively, after one or more of B₂H₆, PH₃, CH₄, N₂, NH₃,AlH₃, AlCl₃, SiH₄, Si₂H₆, F₂, HF, and H₂ are added to the multilayerfilm 107 and the oxide semiconductor film 108, a rare gas may be addedto the multilayer film 107 and the oxide semiconductor film 108.

The addition of the impurity element 125 is controlled by appropriatelysetting the implantation conditions such as the acceleration voltage andthe dose. For example, in the case where argon is added by an ionimplantation method, the acceleration voltage is set to 10 kV and thedose is set to greater than or equal to 1×10¹³ ions/cm² and less than orequal to 1×10¹⁶ ions/cm², e.g., 1×10¹⁴ ions/cm². In the case where aphosphorus ion is added by an ion implantation method, the accelerationvoltage is set to 30 kV and the dose is set to greater than or equal to1×10¹³ ions/cm² and less than or equal to 5×10¹⁶ ions/cm², e.g., 1×10¹⁵ions/cm².

As a result, the low-resistance regions 107 b and 107 c can be formed inthe multilayer film 107. In addition, the low-resistance regions 108 band 108 c can be formed in the oxide semiconductor film 108. After that,the masks 122 and 123 are removed.

Note that when the impurity element 125 is added with the conductivefilms 119 and 120 exposed, part of the conductive films 119 and 120 areseparated and attached to side surfaces of the insulating films 116 and117, This results in an increase in the leakage current of thetransistors. Hence, the impurity element 125 is added to the multilayerfilm 107 and the oxide semiconductor film 108 with the conductive films119 and 120 covered with the masks 122 and 123; thus, it is possible toprevent attachment of part of the conductive films 119 and 120 to theside surfaces of the insulating films 116 and 117.

After that, heat treatment may be performed to further increase theconductivity of the regions to which the impurity element 125 is added.The heat treatment is performed typically at a temperature higher thanor equal to 150° C. and lower than the strain point of the substrate,higher than or equal to 250° C. and lower than or equal to 450° C., orhigher than or equal to 300° C. and lower than or equal to 450° C.

Next, as shown in FIG. 5C, the insulating film 126 is formed over theinsulating film 104, the multilayer film 107, the oxide semiconductorfilm 108, the insulating films 116 and 117, and the conductive films 119and 120.

As a method for forming the insulating film 126, a sputtering method, aCVD method, a vacuum evaporation method, a pulsed laser deposition (PLD)method, or the like is given. Note that a silicon nitride filmcontaining hydrogen can be formed by a plasma CVD method using silaneand ammonia as a source gas or using silane and nitrogen as a sourcegas. Furthermore, by using a plasma CVD method, the multilayer film 107and the oxide semiconductor film 108 can be damaged, and oxygen vacancycan be formed in the multilayer film 107 and the oxide semiconductorfilm 108.

Since hydrogen is contained in the insulating film 126, when theinsulating film 126 is in contact with the regions to which the impurityelement is added in the multilayer film 107 and the oxide semiconductorfilm 108, hydrogen contained in the insulating film 126 moves to theregions to which the impurity element is added in the multilayer film107 and the oxide semiconductor film 108. Since oxygen vacancy isincluded in the regions to which the impurity element is added, thelow-resistance regions can be formed in the multilayer film 107 and theoxide semiconductor film 108.

Alternatively, an aluminum film or an aluminum oxide film is formedinstead of the insulating film 126 and then heat treatment is performed,whereby oxygen contained in the multilayer film 107 and the oxidesemiconductor film 108 reacts with the aluminum film or the aluminumoxide film. Thus, an aluminum oxide film is formed as the insulatingfilm 126, and an oxygen vacancy is formed in the low-resistance regions107 b, 107 c, 108 b, and 108 c in the multilayer film 107 and the oxidesemiconductor film 108. As a result, the conductivity of thelow-resistance regions 107 b, 107 e, 108 b, and 108 c can be furtherincreased.

Here, a 100-nm-thick silicon nitride film is formed as the insulatingfilm 126 by a plasma CVD method.

After that, heat treatment may be performed to further increase theconductivity of the low-resistance regions 107 b, 107 c, 108 b, and 108c. The heat treatment is performed typically at a temperature higherthan or equal to 150° C. and lower than the strain point of thesubstrate, higher than or equal to 250° C. and lower than or equal to450° C., or higher than or equal to 300° C. and lower than or equal to450° C.

Next, the insulating film 127 may be formed as illustrated in FIG. 6A.The insulating film 127 can reduce the parasitic capacitance between theconductive film 119 and the conductive films 134 and 135 formed laterand between the conductive film 120 and the conductive films 136 and 137formed later.

Next, openings 128, 129, 130, and 131 are formed in the insulating films126 and 127 to expose parts of the low-resistance regions, and then theconductive films 134, 135, 136, and 137 are formed. In addition, thenitride insulating film 162 is preferably formed (see FIG. 6B).

The conductive films 134, 135, 136, and 137 can be formed by a methodsimilar to the formation method of the conductive films 119 and 120 asappropriate. The nitride insulating film 162 can be formed by asputtering method, a CVD method, or the like as appropriate.

Through the above-described process, the transistors 100 g and 100 h canbe manufactured.

<Method 2 for Manufacturing Semiconductor Device>

Next, a method for manufacturing the transistors 100 k and 100 zillustrated in FIGS. 8A and 8B is described.

The insulating film 161 is formed over the substrate 101 in the drivercircuit portion, and the conductive film 102 is formed over theinsulating film 161, The conductive film 102 can be formed by theformation method of the conductive films 119 and 120 as appropriate.

Then, the insulating film 104 a and the insulating film 104 b arestacked to form the insulating film 104 over the insulating film 161 andthe conductive film 102.

After that, the multilayer film 107 and the oxide semiconductor film 108are formed through the steps of FIGS. 4A and 4B.

Next, as shown in FIG. 4C, the insulating film 115 is formed, and thenthe insulating film 115 is partly etched, so that an opening 113 shownin FIG. 7A is formed.

Then, the conductive films 119 and 120 shown in FIG. 4C are formed, andthen steps similar to those in FIGS. 5A to 5C and FIGS. 6A and 6B areperformed. In this manner, the transistors 100 k and 100 z can bemanufactured.

In the transistor described in this embodiment, the conductive filmsfunctioning as a source electrode and a drain electrode do not overlapwith the conductive film functioning as a gate electrode, and thus,parasitic capacitance can be reduced and on-state current is high.Furthermore, in the transistor described in this embodiment, thelow-resistance region can be formed stably; therefore, on-state currentis higher and variation in the electrical characteristics of thetransistor is more reduced than in a conventional transistor.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in the other embodiments.

Embodiment 2

In this embodiment, one embodiment of a semiconductor device and amanufacturing method thereof will be described with reference to FIGS.11A to 11C, FIG. 12, FIG. 13, FIG. 14, FIG. 15, FIG. 16, FIG. 17, FIGS.18A to 18C, FIG. 19, FIG. 20, FIGS. 21A to 21C, FIG. 22, FIG. 23, FIGS.24A and 24B, FIGS. 25A and 25B, FIGS. 26A and 26B, and FIGS. 27A and27B.

<Structure 1 of Semiconductor Device>

In FIGS. 11A to 11C, FIG. 12, and FIG. 13, transistors each having atop-gate structure are shown as examples of transistors included in asemiconductor device. Here, a display device is described as an exampleof the semiconductor device. Furthermore, structures of transistorsprovided in a driver circuit portion and a pixel portion of the displaydevice are described.

FIGS. 11A to 11C are top views of a transistor 100 s provided in adriver circuit portion and transistors 100 t and 100 u provided in apixel portion. FIG. 12 and FIG. 13 show cross-sectional views of thetransistors 100 s, 100 t, and 100 u. FIG. 11A, FIG. 11B, and FIG. 11Care top views of the transistor 100 s, the transistor 100 t, and thetransistor 100 u, respectively. FIG. 12 shows cross-sectional viewsalong the dashed-dotted line A-B in FIG. 11A, the dashed-dotted line C-Din FIG. 11B, and the dashed-dotted line E-F in FIG. 11C. FIG. 13 showscross-sectional views along the dashed-dotted line G-H in FIG. 11A, thedashed-dotted line I-J in FIG. 11B, and the dashed-dotted line K-L inFIG. 11C.

The transistor 100 s shown in FIG. 12 and FIG. 13 includes theinsulating film 104 over the substrate 101, the multilayer film 107 overthe insulating film 104, the insulating film 116 in contact with themultilayer film 107, and the conductive film 119 overlapping with themultilayer film 107 with the insulating film 116 provided therebetween.The transistor 100 s has the same structure as the transistor 100 gdescribed in Embodiment 1; therefore, the description of the transistor100 g in Embodiment 1 can be referred to for detailed description of thetransistor 100 s.

The transistor 100 t includes the oxide semiconductor film 108 over theinsulating film 104 formed over the substrate 101; the insulating film117 in contact with the oxide semiconductor film 108; and the conductivefilm 120 overlapping with the oxide semiconductor film 108 with theinsulating film 117 provided therebetween. The transistor 100 t has thesame structure as the transistor 100 h described in Embodiment 1;therefore, the description of the transistor 100 h in Embodiment 1 canbe referred to for detailed description of the transistor 100 t.

The transistor 100 u includes a multilayer film 147 over the insulatingfilm 104 formed over the substrate 101; an insulating film 118 incontact with the multilayer film 147; and a conductive film 121overlapping with the multilayer film 147 with the insulating film 118provided therebetween. The transistor 100 u has the same structure asthe transistor 100 s.

The conductive film 121 functions as a gate electrode. The insulatingfilm 118 functions as a gate insulating film.

The multilayer film 147 includes a channel region 147 a overlapping withthe conductive film 121 and low-resistance regions 147 b and 147 cbetween which the channel region 147 a is positioned. The channel region147 a includes a channel region 145 a in contact with the insulatingfilm 104 and a channel region 146 a in contact with the channel region145 a. The low-resistance region 147 b includes a low-resistance region145 b in contact with the insulating film 104 and a low-resistanceregion 146 b in contact with the low-resistance region 145 b. Thelow-resistance region 147 c includes a low-resistance region 145 c incontact with the insulating film 104 and a low-resistance region 146 cin contact with the low-resistance region 145 c. Note that although notshown in FIG. 12, an oxide semiconductor film including the channelregion 145 a, the low-resistance region 145 b, and the low-resistanceregion 145 c is referred to as an oxide semiconductor film 145, and anoxide semiconductor film including the channel region 146 a, thelow-resistance region 146 b, and the low-resistance region 146 c isreferred to as an oxide semiconductor film 146. That is, the multilayerfilm 147 is a stack including the oxide semiconductor film 145 and theoxide semiconductor film 146.

Note that in a top surface shape, an edge portion of the oxidesemiconductor film 146 is positioned outside an edge portion of theoxide semiconductor film 145. That is, the oxide semiconductor film 146covers a top surface and a side surface of the oxide semiconductor film145.

In the transistor 100 u, the insulating film 126 in contact with thelow-resistance regions 147 b and 147 c is provided. Furthermore, theinsulating film 127 may be provided over the insulating film 126. Inaddition, conductive films 138 and 139 which are in contact with thelow-resistance regions 147 b and 147 c of the multilayer film 147through openings 132 and 133 in the insulating films 126 and 127 areprovided.

In the multilayer film 147, an element which forms an oxygen vacancy isincluded in a region which does not overlap with the conductive film121. As the element which forms an oxygen vacancy, any of the impurityelements given in Embodiment 1 can be used.

The insulating film 126 is a film containing hydrogen, and a nitrideinsulating film is a typical example thereof. Examples of a nitrideinsulating film include a silicon nitride film and an aluminum nitridefilm. The insulating film 126 is in contact with the multilayer film147. Therefore, hydrogen contained in the insulating film 126 isdiffused into the multilayer film 147. As a result, much hydrogen iscontained in a region in contact with the insulating film 126 in themultilayer film 147.

When the impurity element is added to the oxide semiconductor, a bondbetween a metal element and oxygen in the oxide semiconductor is cut,whereby an oxygen vacancy is formed. When hydrogen is added to an oxidesemiconductor in which an oxygen vacancy is formed by addition of animpurity element, hydrogen enters an oxygen vacant site and forms adonor level in the vicinity of the conduction band; thus, theconductivity of the oxide semiconductor is increased. Consequently, anoxide conductor can be formed. Accordingly, the oxide conductor has alight-transmitting property.

The oxide conductor is a degenerate semiconductor and it is suggestedthat the conduction band edge equals to or substantially equals to theFermi level. For that reason, an ohmic contact is made between an oxideconductor film and conductive films functioning as a source electrodeand a drain electrode; thus, contact resistance between the oxideconductor film and the conductive films functioning as a sourceelectrode and a drain electrode can be reduced.

In other words, the low-resistance regions 147 b and 147 c function as asource region and a drain region.

In the case where the conductive films 138 and 139 are formed using aconductive material which is easily bonded to oxygen, such as tungsten,titanium, aluminum, copper, molybdenum, chromium, tantalum, an alloy ofany of these, or the like, oxygen contained in the oxide semiconductorfilms is bonded to the conductive material contained in the conductivefilms 138 and 139, and an oxygen vacancy is formed in the multilayerfilm 147. Furthermore, in some cases, part of constituent elements ofthe conductive material that forms the conductive films 138 and 139 ismixed into the multilayer film 147. As a result, the low-resistanceregions 147 b and 147 c in contact with the conductive films 138 and 139have higher conductivity and function as a source region and a drainregion.

In the case where the impurity element is a rare gas element and themultilayer film 147 is formed by a sputtering method, the low-resistanceregions 147 b and 147 c each contain a rare gas element. In addition,the rare gas element concentrations of the low-resistance regions 147 band 147 c are higher than that of the channel region 147 a. The reasonsare as follows: in the case where the multilayer film 147 is formed by asputtering method, a rare gas is used as a sputtering gas, so that themultilayer film 147 contains the rare gas; and a rare gas isintentionally added to the low-resistance regions 147 b and 147 c inorder to form oxygen vacancies in the low-resistance regions 147 b and147 c. Note that a rare gas element different from that added to thechannel region 147 a may be added to the low-resistance regions 147 band 147 c.

Since the low-resistance regions 147 b and 147 c are in contact with theinsulating film 126, the concentration of hydrogen in the low-resistanceregions 147 b and 147 c is higher than the concentration of hydrogen inthe channel region 147 a.

In the low-resistance regions 147 b and 147 c, the concentration ofhydrogen which is measured by SIMS can be higher than or equal to 8×10¹⁹atoms/cm³, higher than or equal to 1×10²⁰ atoms/cm³, or higher than orequal to 5×10²⁰ atoms/cm³, Note that in the channel region 147 a, theconcentration of hydrogen which is measured by SIMS can be lower than orequal to 5×10¹⁹ atoms/cm³, lower than or equal to 1×10¹⁹ atoms/cm³,lower than or equal to 5×10¹⁸ atoms/cm³, lower than or equal to 1×10¹⁸atoms/cm³, lower than or equal to 5×10¹⁷ atoms/cm³, or lower than orequal to 1×10¹⁶ atoms/cm³.

The low-resistance regions 147 h and 147 c have higher hydrogenconcentrations than the channel region 147 a and have more oxygenvacancies than the channel region 147 a because of addition of rare gaselements. Therefore, the low-resistance regions 147 b and 147 c havehigher conductivity and function as a source region and a drain region.The resistivity of the low-resistance regions 147 b and 147 c can betypically greater than or equal to 1×10⁻³ Ωcm and less than 1×10⁴ Ωcm,or greater than or equal to 1×10⁻³ Ωcm and less than 1×10⁻¹ Ωcm.

Note that in the low-resistance regions 147 b and 147 c, when the amountof hydrogen is smaller than or equal to the amount of oxygen vacancy,hydrogen is easily captured by the oxygen vacancy and is not easilydiffused into the channel region 147 a. As a result, a normally-offtransistor can be manufactured.

Furthermore, in the case where the amount of oxygen vacancy is largerthan the amount of hydrogen in the low-resistance regions 147 b and 147c, the carrier density of the low-resistance regions 147 b and 147 c canbe controlled by controlling the amount of hydrogen. Alternatively, inthe case where the amount of hydrogen is larger than the amount ofoxygen vacancy in the low-resistance regions 147 b and 147 c, thecarrier density of the low-resistance regions 147 b and 147 c can becontrolled by controlling the amount of oxygen vacancy. Note that whenthe carrier density of the low-resistance regions 147 b and 147 c isgreater than or equal to 5×10¹⁸/cm³, greater than or equal to1×10¹⁹/cm³, or greater than or equal to 1×10²⁰/cm³, in the transistor,the resistance between the channel region and the conductive films 138and 139 functioning as source and drain electrodes is small and highon-state current can be obtained.

In the transistors 100 s, 100 t, and 100 u described in this embodiment,the low-resistance regions 107 b and 107 c are provided between thechannel region 107 a and the conductive films 134 and 135 functioning assource and drain electrodes, the low-resistance regions 108 b and 108 care provided between the channel region 108 a and the conductive films136 and 137 functioning as source and drain electrodes, and thelow-resistance regions 147 b and 147 c are provided between the channelregion 147 a and the conductive films 138 and 139 functioning as sourceand drain electrodes; therefore, the transistors have small parasiticresistance.

Furthermore, in the transistor 100 s, the conductive film 119 does notoverlap with the conductive films 134 and 135; therefore, parasiticcapacitance between the conductive film 119 and each of the conductivefilms 134 and 135 can be reduced. In the transistor 100 t, theconductive film 120 does not overlap with the conductive films 136 and137; therefore, parasitic capacitance between the conductive film 120and each of the conductive films 136 and 137 can be reduced. In thetransistor 100 u, the conductive film 121 does not overlap with theconductive films 138 and 139; therefore, parasitic capacitance betweenthe conductive film 121 and each of the conductive films 138 and 139 canbe reduced. As a result, in the case where a large-area substrate isused as the substrate 101, signal delay in the conductive films 119,120, 121, 134, 135, 136, 137, 138, and 139 can be reduced.

Consequently, the transistors 100 s, 100 t, and 100 u have high on-statecurrent and high field-effect mobility.

In the transistor 100 s, the impurity element is added to the multilayerfilm 107 using the conductive film 119 as a mask. In the transistor 100t, the impurity element is added to the oxide semiconductor film 108using the conductive film 120 as a mask. In the transistor 100 u, theimpurity element is added to the multilayer film 147 using theconductive film 121 as a mask. That is, the low-resistance regions canbe formed in a self-aligned manner.

The transistor 100 s in the driver circuit portion has high on-statecurrent and high field-effect mobility. Therefore, a display device inwhich the area occupied by a driver circuit portion is small can bemanufactured.

By using the transistor with high field-effect mobility, a demultiplexercircuit can be formed in a signal line driver circuit which is anexample of the driver circuit portion. A demultiplexer circuitdistributes one input signal to a plurality of outputs; thus, using thedemultiplexer circuit can reduce the number of input terminals for inputsignals. For example, when one pixel includes a red sub-pixel, a greensub-pixel, and a blue sub-pixel and a demultiplexer circuitcorresponding to each pixel is provided, an input signal can bedistributed by the demultiplexer circuit to be input to each sub-pixel.Consequently, the number of input terminals can be reduced to ⅓.

The transistors 100 t and 100 u having high on-state current areprovided in the pixel portion; thus, signal delay in wirings can bereduced and display unevenness can be suppressed even in a large-sizeddisplay device or a high-resolution display device in which the numberof wirings is increased. Note that the luminance of an EL elementincluded in a light-emitting device is proportional to the amount ofcurrent flowing in a transistor that controls the operation of the ELelement. Therefore, when a transistor with high on-state current andhigh field-effect mobility like the transistor 100 u is used as thetransistor that controls the operation of the EL element, the luminanceof the EL element can be increased. Furthermore, since the transistor100 u has high on-state current, the plane area occupied by thetransistor can be small, which leads to higher degree of freedom in thelocation of the transistor in a pixel. Accordingly, the display devicecan have high resolution.

As described above, when a driver circuit portion is formed using atransistor capable of high-speed operation and a pixel portion is formedusing a transistor with small parasitic capacitance and small parasiticresistance, a high-resolution display device capable of double-framerate driving can be manufactured.

The structure shown in FIG. 12 is described in detail below. Here,details of the structure of the transistor 100 s are mainly described.

In the transistors 100 s, the oxide semiconductor film 105 and the oxidesemiconductor film 106 included in the multilayer film 107 havedifferent compositions. In the transistors 100 u, the oxidesemiconductor film 145 and the oxide semiconductor film 146 included inthe multilayer film 147 have different compositions. The oxidesemiconductor film 105 included in the multilayer film 107 and the oxidesemiconductor film 145 included in the multilayer film 147 have the samecomposition. Furthermore, the oxide semiconductor film 106 included inthe multilayer film 107, the oxide semiconductor film 108, and the oxidesemiconductor film 146 included in the multilayer film 147 have the samecomposition. In other words, the oxide semiconductor film 105 and theoxide semiconductor film 145 are formed at the same time, and the oxidesemiconductor film 106, the oxide semiconductor film 108, and the oxidesemiconductor film 146 are formed at the same time.

A channel of the transistor 100 u is formed in the oxide semiconductorfilm 145. Accordingly, the oxide semiconductor film 145 has a largerthickness than the oxide semiconductor film 146.

The oxide semiconductor film 145 can have a desired thickness within thethickness range of the oxide semiconductor film 105.

The oxide semiconductor films 145 and 146 can be formed using thematerials for the oxide semiconductor films 105, 106, and 108 asappropriate. The oxide semiconductor film 145 has a higher indiumcontent than the oxide semiconductor film 146; therefore, a buriedchannel can be formed in the transistor 100 u. Thus, variations in thethreshold voltage of the transistor 100 u can be reduced and channelresistance can be lowered.

Specifically, the oxide semiconductor film 145 can be formed using thematerial for the oxide semiconductor film 105 as appropriate.

Specifically, the oxide semiconductor film 146 can be formed using thematerial for the oxide semiconductor films 106 and 108 as appropriate.

The transistor 100 u has high field-effect mobility because a channel isformed in the oxide semiconductor film 145 in which the proportion of Inatoms is higher than that of Al (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce, Nd,or Hf) atoms. Typically, the transistor has a field-effect mobility ofgreater than 10 cm²/V·s and less than 60 cm²/V·s, preferably greaterthan or equal to 15 cm²/V·s and less than 50 cm²/V·s. However, theoff-state current of the transistor is increased due to lightirradiation, Therefore, by providing a light-blocking film overlappingwith the transistor 100 u, a transistor with high field-effect mobilityand low off-state current is obtained. Consequently, a transistorcapable of high-speed operation can be formed.

In the multilayer film 147, it is preferable to reduce theconcentrations of silicon or carbon that is one of elements belonging toGroup 14, alkali metal or alkaline earth metal, nitrogen, an impurityelement, and the like. Typically, when the multilayer film 147 hasconcentrations of silicon or carbon that is one of elements belonging toGroup 14, alkali metal or alkaline earth metal, nitrogen, an impurityelement, and the like that are substantially equal to those of themultilayer film 107, the transistor 100 u has positive threshold voltage(normally-off characteristics).

By reducing the impurity elements in the multilayer film 147, inparticular, the channel region 147 a, as in the channel region 107 a,the carrier density of the oxide semiconductor films can be lowered.

Oxide semiconductor films each having a low impurity concentration and alow density of defect states can be used for the multilayer film 147, inwhich case the transistor can have more excellent electricalcharacteristics. Here, the state in which impurity concentration is lowand density of defect states is low (the amount of oxygen vacancy issmall) is referred to as “highly purified intrinsic” or “substantiallyhighly purified intrinsic”. A highly purified intrinsic or substantiallyhighly purified intrinsic oxide semiconductor has few carrier generationsources, and thus has a low carrier density in some cases. Thus, atransistor including the oxide semiconductor film in which a channelregion is formed is likely to have positive threshold voltage(normally-off characteristics). A highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has alow density of defect states and accordingly has low density of trapstates in some cases. Furthermore, a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has anextremely low off-state current; the off-state current can be less thanor equal to the measurement limit of a semiconductor parameter analyzer,i.e., less than or equal to 1×10⁻¹³ A, at a voltage (drain voltage)between a source electrode and a drain electrode of from 1 V to 10 V.Thus, the transistor whose channel region is formed in the oxidesemiconductor film has a small variation in electrical characteristicsand high reliability in some cases.

The oxide semiconductor films 145 and 146 can have any of the crystalstructures for the oxide semiconductor films 105, 106, and 108 asappropriate.

Note that in the multilayer film 147, the channel region 147 a and thelow-resistance regions 147 b and 147 c might differ in crystallinity.These cases are due to damage to the low-resistance regions 147 b and147 c, which lowers their crystallinity, when the impurity element isadded to the low-resistance regions 147 b and 147 c.

<Structure 2 of Semiconductor Device>

Next, another structure of the semiconductor device is described withreference to FIG. 14 and FIG. 15. Here, in a transistor 100 v formed ina driver circuit portion and transistors 100 w and 100 x formed in apixel portion, the conductive films 119, 120, and 121 functioning asgate electrodes each have a stacked-layer structure. FIG. 14 showscross-sectional views of the transistors 100 v, 100 w, and 100 x in thechannel length direction, and FIG. 15 shows cross-sectional views of thetransistors 100 v, 100 w, and 100 x in the channel width direction. Thetransistor 100 v has the same structure as the transistor 100 idescribed in Embodiment 1; therefore, the description of the transistor100 i in Embodiment 1 can be referred to for detailed description of thetransistor 100 v. The transistor 100 w has the same structure as thetransistor 100 j described in Embodiment 1; therefore, the descriptionof the transistor 100 j in Embodiment 1 can be referred to for detaileddescription of the transistor 100 w. The transistor 100 x has the samestructure as the transistor 100 v described in this embodiment.

The conductive film 121 included in the transistor 100 x includes aconductive film 121 a in contact with the insulating film 118 and aconductive film 121 b in contact with the conductive film 121 a. The endportion of the conductive film 121 a is positioned on an outer side thanthe end portion of the conductive film 121 b. In other words, theconductive film 121 a has such a shape that the end portion extendsbeyond the end portion of the conductive film 121 b.

The end portion of the insulating film 118 is positioned on an outerside than the end portion of the conductive film 121 a. In other words,the insulating film 118 has such a shape that the end portion extendsbeyond the end portion of the conductive film 121 a. Furthermore, a sidesurface of the insulating film 118 may be curved.

The conductive film 121 a can be formed using the material for theconductive films 119 a and 120 a as appropriate. The conductive film 121b can be formed using the material for the conductive films 119 b and120 b as appropriate.

Note that when the concentration of copper in the channel region in themultilayer film 147 is within the range given for the multilayer film107, a transistor having an excellent subthreshold swing value (S value)can be manufactured.

As in the transistors 100 v and 100 w, when the conductive film 121 andthe insulating film 118 having the shapes shown in FIG. 14 and FIG. 15are provided in the transistor 100 x, the electric field of the drainregion of the transistor can be relaxed. Thus, deterioration of thetransistor due to the electric field of the drain region, such as ashift of the threshold voltage of the transistor, can be inhibited.

<Structure 3 of Semiconductor Device>

Next, other structures of the semiconductor device are described withreference to FIG. 16 and FIG. 17. Here, transistors 111 a and 111 dformed in a driver circuit portion and transistors 111 b, 111 c, 111 e,and 111 f formed in a pixel portion each include a multilayer film. FIG.16 shows cross-sectional views of the transistors 111 a, 111 b, and 111c in the channel length direction, and FIG. 17 shows cross-sectionalviews of the transistors 111 d, 111 e, and 111 f in the channel lengthdirection.

The transistor 111 a has the same structure as the transistor 111 wdescribed in Embodiment 1; therefore, the description of the transistor111 w in Embodiment 1 can be referred to for detailed description of thetransistor 111 a. The transistor 111 b has the same structure as thetransistor 100 h described in Embodiment 1; therefore, the descriptionof the transistor 100 h in Embodiment 1 can be referred to for detaileddescription of the transistor 111 b. The transistor 111 c has the samestructure as the transistor 111 a described in this embodiment;therefore, the description of the transistor 111 w in Embodiment 1 canbe referred to for detailed description of the transistor 111 c. Thetransistor 111 d has the same structure as the transistor 111 xdescribed in Embodiment 1; therefore, the description of the transistor111 x in Embodiment 1 can be referred to for detailed description of thetransistor 111 d. The transistor 111 e has the same structure as thetransistor 111 y described in Embodiment 1; therefore, the descriptionof the transistor 111 y in Embodiment 1 can be referred to for detaileddescription of the transistor 111 e. The transistor 111 f has the samestructure as the transistor 111 d described in this embodiment;therefore, the description of the transistor 111 x in Embodiment 1 canbe referred to for detailed description of the transistor 111 f.

The multilayer film 147 included in the transistor 111 c in FIG. 16includes the channel region 147 a and the low-resistance regions 147 band 147 c. The channel region 147 a includes a channel region 148 a incontact with the insulating film 104, the channel region 145 a incontact with the channel region 148 a, and the channel region 146 a incontact with the channel region 145 a. The low-resistance region 147 bincludes a low-resistance region 148 b in contact with the insulatingfilm 104, the low-resistance region 145 b in contact with thelow-resistance region 148 b, and the low-resistance region 146 b incontact with the low-resistance region 145 b. The low-resistance region147 c includes a low-resistance region 148 c in contact with theinsulating film 104, the low-resistance region 145 c in contact with thelow-resistance region 148 c, and the low-resistance region 146 c incontact with the low-resistance region 145 c. Note that although notshown in FIG. 16, an oxide semiconductor film including the channelregion 148 a, the low-resistance region 148 b, and the low-resistanceregion 148 c is referred to as an oxide semiconductor film 148. That is,in the multilayer film 147, the oxide semiconductor film 148, the oxidesemiconductor film 145, and the oxide semiconductor film 146 are stackedin this order.

The multilayer film 147 included in the transistor 111 f in FIG. 17includes the channel region 147 a and the low-resistance regions 147 band 147 c. The channel region 147 a includes the channel region 148 a incontact with the insulating film 104, the channel region 145 a incontact with the channel region 148 a, the channel region 146 a incontact with the channel region 145 a, and a channel region 149 a incontact with the channel region 146 a. The low-resistance region 147 bincludes the low-resistance region 148 b in contact with the insulatingfilm 104, the low-resistance region 145 b in contact with thelow-resistance region 148 b, the low-resistance region 146 b in contactwith the low-resistance region 145 b, and a low-resistance region 149 bin contact with the low-resistance region 146 b. The low-resistanceregion 147 c includes the low-resistance region 148 c in contact withthe insulating film 104, the low-resistance region 145 c in contact withthe low-resistance region 148 c, the low-resistance region 146 e incontact with the low-resistance region 145 c, and a low-resistanceregion 149 c in contact with the low-resistance region 146 c. Note thatalthough not shown in FIG. 17, an oxide semiconductor film including thechannel region 149 a, the low-resistance region 149 b, and thelow-resistance region 149 c is referred to as an oxide semiconductorfilm 149. That is, in the multilayer film 147, the oxide semiconductorfilm 148, the oxide semiconductor film 145, the oxide semiconductor film146, and the oxide semiconductor film 149 are stacked in this order.

The oxide semiconductor films 148 and 149 each preferably have a largerenergy gap, a lower electron affinity, and a higher insulating propertythan the oxide semiconductor films 145 and 146. In addition, the oxidesemiconductor films 148 and 149 each preferably have a lower indiumcontent than the oxide semiconductor films 145 and 146. The oxidesemiconductor films 148 and 149 preferably have a function of blockingexternal impurity. In such an oxide semiconductor film, the proportionof In atoms is lower than that of M (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce,Nd, or Hf) atoms. In the case where the oxide semiconductor films 148and 149 contain an In-M-Zn oxide (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce,Nd, or Hf), and a target having the atomic ratio of the metal elementsof In:M:Zn=x₃:y₃:z₃ is used for foaming the oxide semiconductor films148 and 149, x₃/y₃ is preferably greater than or equal to ⅙ and lessthan 1; z₃/y₃ is preferably greater than or equal to ⅓ and less than orequal to 6, further preferably greater than or equal to 1 and less thanor equal to 6. Note that when z₃/y₃ is greater than or equal to 1 andless than or equal to 6, CAAC-OS films are easily formed as the oxidesemiconductor films 148 and 149. Typical examples of the atomic ratio ofthe metal elements of the target are In:M:Zn=1:3:2, In:M:Zn=1:3:4,In:M:Zn=1:3:6, In:M:Zn=1:3:8, In:M:Zn=1:4:4, In:M:Zn=1:4:5,In:M:Zn=1:4:6, In:M:Zn=1:4:7, In:M:Zn=1:4:8, In:M:Zn=1:5:5,In:M:Zn=1:5:6, In:M:Zn=1:5:7, In:M:Zn=1:5:8, and In:M:Zn=1:6:8.

In the transistor 111 a shown in FIG. 16, the oxide semiconductor film142 has a larger energy gap and a lower electron affinity than the oxidesemiconductor film 105; therefore, a channel is formed in the oxidesemiconductor film 105. That is, a buried channel structure is obtained.The oxide semiconductor film 106 and the oxide semiconductor film 142contain one or more kinds of metal elements that form the oxidesemiconductor film 105. Therefore, an interface state is not easilyformed at the interface between the oxide semiconductor film 105 and theoxide semiconductor film 106 and at the interface between the oxidesemiconductor film 105 and the oxide semiconductor film 142. Thus,providing the oxide semiconductor film 106 and the oxide semiconductorfilm 142 makes it possible to reduce variations or changes in electricalcharacteristics of the transistor, such as threshold voltage. Note thata similar effect is obtained with the transistor 111 c.

Similarly, in the transistor 111 d shown in FIG. 17, the oxidesemiconductor films 142 and 143 each have a larger energy gap and alower electron affinity than the oxide semiconductor films 105 and 106;therefore, a channel is formed in the oxide semiconductor film 105. Thatis, a buried channel structure is obtained. The multilayer film 107includes the oxide semiconductor films 142 and 143. Therefore, aninterface state is not easily formed at the interface between the oxidesemiconductor film 142 and the oxide semiconductor film 105, at theinterface between the oxide semiconductor film 105 and the oxidesemiconductor film 106, and at the interface between the oxidesemiconductor film 106 and the oxide semiconductor film 143.Consequently, variations or changes in electrical characteristics of thetransistor, such as threshold voltage, can be reduced. Note that asimilar effect is obtained with the transistor 111 f.

Similarly, in the transistor 111 e shown in FIG. 17, the multilayer film110 includes the oxide semiconductor film 144; therefore, an interfacestate is not easily formed at the interface between the oxidesemiconductor film 144 and the oxide semiconductor film 108,Consequently, variations or changes in electrical characteristics of thetransistor, such as threshold voltage, can be reduced.

<Structure 4 of Semiconductor Device>

Next, another structure of the semiconductor device is described withreference to FIGS. 18A to 18C, FIG. 19, and FIG. 20. Here, a transistor111 g formed in a driver circuit portion and a transistor 111 i formedin a pixel portion each have a dual-gate structure. FIG. 18A, FIG. 18B,and FIG. 18C are top views of the transistor 111 g, a transistor 111 h,and the transistor 111 i, respectively. FIG. 19 shows cross-sectionalviews along the dashed-dotted line A-B in FIG. 18A, the dashed-dottedline C-D in FIG. 18B, and the dashed-dotted line E-F in FIG. 18C. FIG.20 shows cross-sectional views along the dashed-dotted line G-H in FIG.18A, the dashed-dotted line in FIG. 18B, and the dashed-dotted line K-Lin FIG. 18C.

The transistor 111 g shown in FIG. 19 and FIG. 20 includes theconductive film 102 over the substrate 101, the insulating film 104 overthe substrate 101 and the conductive film 102, the multilayer film 107over the insulating film 104, the insulating film 116 in contact withthe multilayer film 107, and the conductive film 119 overlapping withthe multilayer film 107 with the insulating film 116 providedtherebetween. The structure of the multilayer film 107 is similar tothat of the multilayer film 107 described in <Structure 1 ofsemiconductor device> and therefore is not described in detail here.

The conductive films 102 and 119 function as gate electrodes. That is,the transistor 111 g is a transistor having a dual-gate structure. Theinsulating films 104 and 116 function as gate insulating films.

The transistor 111 h can have a structure similar to that of thetransistor 100 t described in <Structure 1 of semiconductor device> andtherefore is not described in detail here.

The transistor 111 i shown in FIG. 19 and FIG. 20 includes a conductivefilm 103 over the substrate 101, the insulating film 104 over thesubstrate 101 and the conductive film 103, the multilayer film 147 overthe insulating film 104, the insulating film 118 in contact with themultilayer film 147, and the conductive film 121 overlapping with themultilayer film 147 with the insulating film 118 provided therebetween.The structure of the multilayer film 147 is similar to that of themultilayer film 147 described in <Structure 1 of semiconductor device>and therefore is not described in detail here.

The nitride insulating film 161 is preferably provided over thesubstrate 101. Examples of the nitride insulating film 161 include asilicon nitride film and an aluminum nitride film. Covering theconductive films 102 and 103 with the nitride insulating film 161 andthe insulating film 104 a makes it possible to prevent diffusion ofmetal elements contained in the conductive films 102 and 103.

The conductive films 103 and 121 function as gate electrodes. That is,the transistor 111 i is a transistor having a dual-gate structure. Theinsulating films 104 and 118 function as gate insulating films.

Note that the conductive films 102 and 103 overlap with the entiremultilayer film 107 and the entire multilayer film 147, respectively, inFIG. 19 and FIG. 20; however, the conductive films 102 and 103 mayoverlap with part of the multilayer film 107 and part of the multilayerfilm 147, respectively.

In the transistor 111 g, different potentials are supplied to theconductive film 102 and the conductive film 119 which are not connectedto each other; thus, the threshold voltage of the transistor 111 g canbe controlled. Alternatively, as shown in FIG. 20, by supplying the samepotential to the conductive film 102 and the conductive film 119 whichare connected to each other, variations in the initial characteristicscan be reduced, and degradation of the transistor due to the −GBT(negative gate bias-temperature) stress test and a change in the risingvoltage of the on-state current at different drain voltages can besuppressed. Furthermore, when the conductive film 102 and the conductivefilm 119 are connected to each other as shown in FIG. 20, electricfields of the conductive films 102 and 119 affect a top surface and aside surface of the multilayer film 107, so that carriers flow in theentire multilayer film 107. In other words, a region where carriers flowbecomes larger in the film thickness direction, so that the amount ofcarrier movement is increased. As a result, the on-state current andfield-effect mobility of the transistor 111 g are increased. When thechannel length of the transistor is less than 2.5 μm, preferably greaterthan or equal to 1.45 μm and less than or equal to 2.2 μm, the on-statecurrent can be further increased and the field-effect mobility can beincreased. Owing to the high on-state current, the transistor 111 g canhave a small plane area. Consequently, a display device with a narrowbezel in which the area occupied by a driver circuit portion is smallcan be manufactured.

Note that an effect similar to that obtained with the transistor 111 gcan be obtained when the conductive film 103 and the conductive film 121are connected to each other in the transistor 111 i. Thus, the planearea occupied by the transistor can be small, which leads to higherdegree of freedom in the location of the transistor in a pixel.Accordingly, the display device can have high resolution.

In the display device described in this embodiment, the transistorincluded in the driver circuit portion has a dual-gate structure. Thatis, the field-effect mobility of the transistor included in the drivercircuit portion is higher than that of the transistor included in thepixel portion. Consequently, a driver circuit portion capable ofhigh-speed operation can be formed. Furthermore, when a driver circuitportion is formed using a transistor capable of high-speed operation anda pixel portion is formed using a transistor with small parasiticcapacitance and small parasitic resistance, a high-resolution displaydevice capable of double-frame rate driving can be manufactured.

<Structure 5 of Semiconductor Device>

Next, another structure of the semiconductor device is described withreference to FIGS. 21A to 21C, FIG. 22, and FIG. 23. Here, a transistor111 m formed in a pixel portion has a dual-gate structure. FIG. 21A,FIG. 21B, and FIG. 21C are top views of a transistor 111 j, a transistor111 k, and the transistor 111 m, respectively. FIG. 22 showscross-sectional views along the dashed-dotted line A-B in FIG. 21A, thedashed-dotted line C-D in FIG. 21B, and the dashed-dotted line E-F inFIG. 21C. FIG. 23 shows cross-sectional views along the dashed-dottedline G-H in FIG. 21A, the dashed-dotted line in FIG. 21B, and thedashed-dotted line K-L in FIG. 21C.

The transistor 111 j in FIG. 22 and FIG. 23 has a single-gate structurelike the transistor 100 s described in <Structure 1 of semiconductordevice> and therefore is not described in detail here.

The transistor 111 k in FIG. 22 and FIG. 23 has the same structure asthe transistor 100 t described in <Structure 1 of semiconductor device>and therefore is not described in detail here.

The transistor 111 m in FIG. 22 and FIG. 23 has a dual-gate structurelike the transistor 111 i described in <Structure 4 of semiconductordevice> and therefore is not described in detail here.

In the display device described in this embodiment, the transistor(e.g., the transistors 111 i and 111 m) provided in the pixel portionincludes the conductive films 103 and 121 which block entry of light tothe multilayer film 147. Thus, the transistor (e.g., the transistors 111i and 111 m) has high field-effect mobility and low off-state current.Therefore, the plane area occupied by the transistor can be small, whichleads to higher degree of freedom in the location of the transistor in apixel, and deterioration of image quality can be suppressed.Accordingly, the display device can have high image quality and highresolution. Furthermore, when the pixel portion is formed using atransistor with small parasitic capacitance and small parasiticresistance, a high-resolution display device capable of double-framerate driving can be manufactured.

<Method 1 for manufacturing semiconductor device>

Next, a method for manufacturing the transistors 100 s, 100 t, and 100 uillustrated in FIGS. 11A to 11C, FIG. 12, and FIG. 13 will be describedwith reference to FIGS. 24A and 24B, FIGS. 25A and 25B, FIGS. 26A and26B, and FIGS. 27A and 27B.

The films included in the transistors 100 s, 100 t, and 100 u (i.e., theinsulating film, the oxide semiconductor film, the conductive film, andthe like) can be formed by the formation methods of the films includedin the transistors described in Embodiment 1 as appropriate.

As shown in FIG. 24A, the insulating film 104 is formed over thesubstrate 101, in a manner similar to that in Embodiment 1. Next, theoxide semiconductor film 105 is formed over the insulating film 104 inthe driver circuit portion, and the oxide semiconductor film 145 isformed over the insulating film 104 in the pixel portion.

Here, a 300-nm-thick silicon oxynitride film is formed as the insulatingfilm 104 by a plasma CVD method.

The oxide semiconductor films 105, 106, 108, 145, and 146 can be formedin manners similar to those of the oxide semiconductor films 105, 106,and 108 described in Embodiment 1.

Furthermore, after the oxide semiconductor films are formed, heattreatment may be performed so that the oxide semiconductor films aresubjected to dehydrogenation or dehydration, in a manner similar to thatin Embodiment 1.

Here, a 35-nm-thick oxide semiconductor film is formed by a sputteringmethod. Next, a mask is formed over the oxide semiconductor film, andpart of the oxide semiconductor film is selectively etched. In thismanner, the oxide semiconductor films 105 and 145 are formed. As theoxide semiconductor film, an In—Ga—Zn oxide film (In:Ga:Zn=3:1:2) isformed.

Next, as shown in FIG. 24B, the oxide semiconductor film 106 is formedover the oxide semiconductor film 105 in the driver circuit portion, andthe oxide semiconductor film 108 is formed and the oxide semiconductorfilm 146 is formed over the oxide semiconductor film 145 in the pixelportion. Thus, the multilayer film 107 in which the oxide semiconductorfilm 105 and the oxide semiconductor film 106 are stacked in this orderis formed. In addition, the multilayer film 147 in which the oxidesemiconductor film 145 and the oxide semiconductor film 146 are stackedin this order is formed.

Note that in this step, the oxide semiconductor film 106 is formed tocover a top surface and a side surface of the oxide semiconductor film105, to prevent the oxide semiconductor film 105 from being etched in alater step of forming conductive films functioning as a source electrodeand a drain electrode. In addition, the oxide semiconductor film 146 isformed to cover a top surface and a side surface of the oxidesemiconductor film 145, to prevent the oxide semiconductor film 145 frombeing etched in a later step of forming conductive films functioning asa source electrode and a drain electrode. This is preferable becausevariations in the lengths of the oxide semiconductor films 105 and 145in the channel width direction of the transistors can be reduced.

Here, a 20-nm-thick oxide semiconductor film is formed by a sputteringmethod, Next, a mask is formed over the oxide semiconductor film, andpart of the oxide semiconductor film is selectively etched. In thismanner, the oxide semiconductor films 106, 108, and 146 are formed. Asthe oxide semiconductor films 106, 108, and 146, In—Ga—Zn oxide films(In:Ga:Zn=1:1:1.2) are formed.

Next, oxygen contained in the insulating film 104 is moved to the oxidesemiconductor films by heat treatment. Note that the heat treatment maybe performed at a time that is after the formation of the oxidesemiconductor film to be the oxide semiconductor films 106, 108, and 146and before the etching of the oxide semiconductor film for fainting theoxide semiconductor films 106, 108, and 146.

When the heat treatment is performed at a temperature higher than 350°C. and lower than or equal to 650° C., or higher than or equal to 450°C. and lower than or equal to 600° C., it is possible to obtain an oxidesemiconductor film whose proportion of CAAC, which is described later,is greater than or equal to 60% and less than 100%, greater than orequal to 80% and less than 100%, greater than or equal to 90% and lessthan 100%, or greater than or equal to 95% and less than or equal to98%. Furthermore, it is possible to obtain an oxide semiconductor filmhaving a low content of hydrogen, water, and the like. That is, an oxidesemiconductor film with a low impurity concentration and a low densityof defect states can be formed.

Next, as shown in FIG. 25A, the insulating film 115 is formed over theinsulating film 104, the multilayer films 107 and 147, and the oxidesemiconductor film 108, in a manner similar to that in Embodiment 1.Then, the conductive films 119, 120, and 121 are formed over theinsulating film 115, in a manner similar to that in Embodiment 1.

Here, a 100-nm-thick silicon oxynitride film is formed by a plasma CVDmethod as the insulating film 115.

Here, masks 122, 123, and 124 are formed over the conductive film by alithography process and then the conductive film is etched, whereby theconductive films 119, 120, and 121 are formed.

Then, as shown in FIG. 25B, the insulating film 115 is etched with themasks 122, 123, and 124 left, so that the insulating films 116, 117, and118 are formed, in a manner similar to that in Embodiment 1.

Next, as shown in FIG. 26A, the impurity element 125 is added to themultilayer films 107 and 147 and the oxide semiconductor film 108 withthe masks 122, 123, and 124 left, in a manner similar to that inEmbodiment 1. As a result, the impurity element is added to regionswhich are not covered with the masks 122, 123, and 124 in the multilayerfilms 107 and 147 and the oxide semiconductor film 108. Note that by theaddition of the impurity element 125, an oxygen vacancy is formed in themultilayer films 107 and 147 and the oxide semiconductor film 108.

As a result, the low-resistance regions 107 b and 107 c can be formed inthe multilayer film 107. The low-resistance regions 108 b and 108 c canbe formed in the oxide semiconductor film 108. In addition, thelow-resistance regions 147 b and 147 e can be formed in the multilayerfilm 147. After that, the masks 122, 123, and 124 are removed.

Note that when the impurity element 125 is added with the conductivefilms 119, 120, and 121 exposed, part of the conductive films 119, 120,and 121 are separated and attached to side surfaces of the insulatingfilms 116, 117, and 118. This results in an increase in the leakagecurrent of the transistors. Hence, the impurity element 125 is added tothe multilayer films 107 and 147 and the oxide semiconductor film 108with the conductive films 119, 120, and 121 covered with the masks 122,123, and 124; thus, it is possible to prevent attachment of part of theconductive films 119, 120, and 121 to the side surfaces of theinsulating films 116, 117, and 118. Alternatively, the impurity element125 may be added to the multilayer films 107 and 147 and the oxidesemiconductor film 108 after the masks 122, 123, and 124 are removed.

After that, heat treatment may be performed to further increase theconductivity of the regions to which the impurity element 125 is added,in a manner similar to that in Embodiment 1.

Next, as shown in FIG. 27A, the insulating film 126 is formed over theinsulating film 104, the multilayer films 107 and 147, the oxidesemiconductor film 108, the insulating films 116, 117, and 118, and theconductive films 119, 120, and 121, in a manner similar to that inEmbodiment 1.

Here, a 100-nm-thick silicon nitride film is formed as the insulatingfilm 126 by a plasma CVD method.

After that, heat treatment may be performed to further increase theconductivity of the low-resistance regions 107 b, 107 c, 108 b, 108 c,147 b, and 147 c, in a manner similar to that in Embodiment 1. The heattreatment is performed typically at a temperature higher than or equalto 150° C. and lower than the strain point of the substrate, higher thanor equal to 250° C. and lower than or equal to 450° C., or higher thanor equal to 300° C. and lower than or equal to 450° C.

Next, the insulating film 127 may be forming as illustrated in FIG. 27A,in a manner similar to that in Embodiment 1. The insulating film 127 canreduce the parasitic capacitance between the conductive film 119 and theconductive films 134 and 135 formed later, between the conductive film120 and the conductive films 136 and 137 formed later, and between theconductive film 121 and the conductive films 138 and 139 formed later.

Next, in a manner similar to that in Embodiment 1, openings 128, 129,130, 131, 132, and 133 (see FIGS. 21A to 21C) are formed in theinsulating films 126 and 127 to expose parts of the low-resistanceregions, and then the conductive films 134, 135, 136, 137, 138, and 139are formed. In addition, the nitride insulating film 162 is preferablyformed (see FIG. 27B).

The conductive films 134, 135, 136, 137, 138, and 139 can be formed by amethod similar to the formation method of the conductive films 119 and120 as appropriate. The nitride insulating film 162 can be formed by asputtering method, a CVD method, or the like as appropriate.

Through the above-described process, the transistors 100 s, 100 t, and100 u can be manufactured.

<Method 2 for Manufacturing Semiconductor Device>

Next, a method for manufacturing the transistors 111 g, 111 h, and 111 iillustrated in FIGS. 18A to 18C, FIG. 19, and FIG. 20 is described.

The insulating film 161 is thrilled over the substrate 101, and theconductive films 102 and 103 are formed over the insulating film 161.The conductive films 102 and 103 can be formed by the formation methodof the conductive films 119 and 120 as appropriate.

Then, the insulating film 104 is formed over the insulating film 161 andthe conductive films 102 and 103.

After that, the multilayer films 107 and 147 and the oxide semiconductorfilm 108 are formed through the steps of FIGS. 24A and 24B.

Next, as shown in FIG. 25A, the insulating film 115 is formed, and thenthe insulating film 115 is partly etched, so that the opening 113 shownin FIG. 18A and an opening 114 shown in FIG. 18C are formed.

Then, the conductive films 119, 120, and 121 shown in FIG. 25A areformed, and then steps similar to those in FIG. 25B, FIGS. 26A and 26B,and FIGS. 27A and 27B are performed. In this manner, the transistors 111g, 111 h, and 111 i can be manufactured.

In the transistor described in this embodiment, the conductive filmsfunctioning as a source electrode and a drain electrode do not overlapwith the conductive film functioning as a gate electrode, and thus,parasitic capacitance can be reduced and on-state current is high.Furthermore, in the transistor described in this embodiment, thelow-resistance region can be formed stably; therefore, on-state currentis higher and variation in the electrical characteristics of thetransistor is more reduced than in a conventional transistor.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in the other embodiments.

Embodiment 3

In this embodiment, modification examples of the transistors describedin the above embodiments will be described with reference to FIGS. 29Ato 29F, FIGS. 30A to 30F, FIGS. 31A to 31E, FIGS. 32A and 32B, and FIGS.33A to 33D. Here, the transistor formed in the pixel portion isdescribed as a typical example. Transistors illustrated in FIGS. 29A to29F each include the oxide semiconductor film 108 over the insulatingfilm 104 over the substrate 101, the insulating film 117 in contact withthe oxide semiconductor film 108, and the conductive film 120 incontract with the insulating film 117 and overlapping the oxidesemiconductor film 108.

The transistors each include the insulating film 126 that is in contactwith the oxide semiconductor film 108 and the insulating film 127 thatis in contact with the insulating film 126. The conductive films 136 and137 that are in contact with the oxide semiconductor film 108 throughopenings in the insulating film 126 and the insulating film 127 are alsoincluded.

In the transistor illustrated in FIG. 29A, the oxide semiconductor film108 includes the channel region 108 a formed in a region overlappingwith the conductive film 120 and the low-resistance regions 108 b and108 c between which the channel region 108 a is provided and whichcontain the impurity elements. The conductive films 136 and 137 are incontact with the low-resistance regions 108 b and 108 c, respectively.

Alternatively, as in the transistor illustrated in FIG. 29B, an impurityelement is not necessarily added to regions 108 d and 108 e of the oxidesemiconductor film 108 which are in contact with the conductive films136 and 137, respectively. In this case, regions containing the impurityelements, i.e., the low-resistance regions 108 b and 108 c are provided.The low-resistance region (108 b or 108 c) is provided between thechannel region 108 a and the region (108 d or 108 e) in contact with theconductive film (136 or 137). The regions 108 d and 108 e haveconductivity when voltage is applied to the conductive films 136 and137; thus, the regions 108 d and 108 e function as a source region and adrain region.

Note that the transistor illustrated in FIG. 29B can be formed in such amanner that the conductive films 136 and 137 are formed and thenimpurity elements are added to the oxide semiconductor film using theconductive film 120 and the conductive films 136 and 137 as masks.

An end portion of the conductive film 120 may have a tapered shape. Thatis, an angle θ1 formed between a surface where the insulating film 117and the conductive film 120 are in contact with each other and a sidesurface of the conductive film 120 may be less than 90°, greater than orequal to 10° and less than or equal to 85°, greater than or equal to 15°and less than or equal to 85°, greater than or equal to 30° and lessthan or equal to 85°, greater than or equal to 45° and less than orequal to 85°, or greater than or equal to 60° and less than or equal to85°. When the angle θ1 is less than 90°, greater than or equal to 10°and less than or equal to 85°, greater than or equal to 15° and lessthan or equal to 85°, greater than or equal to 30° and less than orequal to 85°, greater than or equal to 45° and less than or equal to85°, or greater than or equal to 60° and less than or equal to 85°, thecoverage of the side surfaces of the insulating film 117 and theconductive film 120 with the insulating film 126 can be improved.

Next, modification examples of the low-resistance regions 108 b and 108c are described. FIGS. 29C to 29F are each an enlarged view of thevicinity of the oxide semiconductor film 108 illustrated in FIG. 29A.The channel length L indicates a distance between a pair oflow-resistance regions.

As illustrated in FIG. 29C, in a cross-sectional view in the channellength direction, the boundaries between the channel region 108 a andthe low-resistance regions 108 b and 108 c are aligned or substantiallyaligned with the end portions of the conductive film 120 with theinsulating film 117 provided therebetween. That is, the boundariesbetween the channel region 108 a and the low-resistance regions 108 band 108 c are aligned or substantially aligned with the end portions ofthe conductive film 120, when seen from the above.

Alternatively, as illustrated in FIG. 29D, in a cross-sectional view inthe channel length direction, the channel region 108 a has a region thatdoes not overlap with the end portion of the conductive film 120. Theregion functions as an offset region. The length of the offset region inthe channel length direction is referred to as L_(off). Note that in thecase where a plurality of offset regions are provided, L_(off) indicatesthe length of one offset region. L_(off) is included in the channellength L. Note that L_(off) is smaller than 20%, smaller than 10%,smaller than 5%, or smaller than 2% of the channel length L.

Alternatively, as illustrated in FIG. 29E, in a cross-sectional view inthe channel length direction, the low-resistance regions 108 b and 108 ceach have a region overlapping with the conductive film 120 with theinsulating film 117 provided therebetween. This region functions as anoverlap region. The overlap region in the channel length direction isreferred to as L_(ov). L_(ov) is smaller than 20%, smaller than 10%,smaller than 5%, or smaller than 2% of the channel length L.

Alternatively, as illustrated in FIG. 29F, in a cross-sectional view inthe channel length direction, a low-resistance region 108 f between thechannel region 108 a and the low-resistance region 108 b, and alow-resistance region 108 g between the channel region 108 a and thelow-resistance region 108 c are provided. The low-resistance regions 108f and 108 g have lower impurity element concentrations and higherresistivity than the low-resistance regions 108 b and 108 c. Here, thelow-resistance regions 108 f and 108 g overlap with the insulating film117, but they may overlap with the insulating film 117 and theconductive film 120.

Note that in FIGS. 29C to 29F, the transistor illustrated in FIG. 29A isdescribed; however, the transistor illustrated in FIG. 29B can employany of the structures in FIGS. 29C to 29F as appropriate.

In the transistor illustrated in FIG. 30A, the end portion of theinsulating film 117 is positioned on an outer side than the end portionof the conductive film 120. In other words, the insulating film 117 hassuch a shape that the end portion extends beyond the end portion of theconductive film 120. The insulating film 126 can be distanced from thechannel region 108 a; thus, nitrogen, hydrogen, and the like containedin the insulating film 126 can be prevented from entering the channelregion 108 a.

In the transistor illustrated in FIG. 30B, the insulating film 117 andthe conductive film 120 each have a tapered shape, and the angles of thetapered shapes are different from each other. In other words, the angleθ1 formed between a surface where the insulating film 117 and theconductive film 120 are in contact with each other and a side surface ofthe conductive film 120 is different from an angle θ2 formed between asurface where the oxide semiconductor film 108 and the insulating film117 are in contact with each other and a side surface of the insulatingfilm 117. The angle θ2 may be less than 90°, greater than or equal to30° and less than or equal to 85°, or greater than or equal to 45° andless than or equal to 70°. For example, when the angle θ2 is smallerthan the angle θ1, the coverage with the insulating film 126 isimproved. In contrast, when the angle θ2 is larger than the angle θ1,the transistor can be miniaturized.

Next, modification examples of the low-resistance regions 108 b and 108c are described with reference to FIGS. 30C to 30F. FIGS. 30C to 30F areeach an enlarged view of the vicinity of the oxide semiconductor film108 illustrated in FIG. 30A.

As illustrated in FIG. 30C, in a cross-sectional view in the channellength direction, the boundaries between the channel region 108 a andthe low-resistance regions 108 b and 108 c are aligned or substantiallyaligned with the end portions of the conductive film 120 with theinsulating film 117 provided therebetween. That is, the boundariesbetween the channel region 108 a and the low-resistance regions 108 band 108 c are aligned or substantially aligned with the end portions ofthe conductive film 120, when seen from the above.

Alternatively, as illustrated in FIG. 30D, in a cross-sectional view inthe channel length direction, the channel region 108 a has a region thatdoes not overlap with the conductive film 120. The region functions asan offset region. That is, when seen from the above, the end portions ofthe low-resistance regions 108 b and 108 c are aligned or substantiallyaligned with the end portions of the insulating film 117 and do notoverlap with the end portions of the conductive film 120.

Alternatively, as illustrated in FIG. 30E, in a cross-sectional view inthe channel length direction, the low-resistance regions 108 b and 108 ceach have a region overlapping with the conductive film 120 with theinsulating film 117 provided therebetween. The region is referred to asan overlap region. That is, when seen from the above, the end portionsof the low-resistance regions 108 b and 108 c overlap with theconductive film 120.

Alternatively, as illustrated in FIG. 30F, in a cross-sectional view inthe channel length direction, the low-resistance region 108 f betweenthe channel region 108 a and the low-resistance region 108 b, and thelow-resistance region 108 g between the channel region 108 a and thelow-resistance region 108 c are provided. The low-resistance regions 108f and 108 g have lower impurity element concentrations and higherresistivity than the low-resistance regions 108 b and 108 c. Here, thelow-resistance regions 108 f and 108 g overlap with the insulating film117, but they may overlap with the insulating film 117 and theconductive film 120.

Note that in FIGS. 30C to 30F, the transistor illustrated in FIG. 30A isdescribed; however, the transistor illustrated in FIG. 30B can employany of the structures in FIGS. 30C to 30F as appropriate.

In the transistor illustrated in FIG. 31A, the conductive film 120 has astacked-layer structure including the conductive film 120 a in contactwith the insulating film 117 and the conductive film 120 b in contactwith the conductive film 120 a. The end portion of the conductive film120 a is positioned on an outer side than the end portion of theconductive film 120 b. In other words, the conductive film 120 a hassuch a shape that the end portion extends beyond the end portion of theconductive film 120 b.

Next, modification examples of the low-resistance regions 108 b and 108c are described. FIGS. 31B to 31E and FIGS. 32A and 32B are each anenlarged view of the vicinity of the oxide semiconductor film 108illustrated in FIG. 31A.

As illustrated in FIG. 31B, in a cross-sectional view in the channellength direction, the boundaries between the channel region 108 a andthe low-resistance regions 108 b and 108 c are aligned or substantiallyaligned with the end portions of the conductive film 120 a in theconductive film 120 with the insulating film 117 provided therebetween.That is, the boundaries between the channel region 108 a and thelow-resistance regions 108 b and 108 c are aligned or substantiallyaligned with the end portions of the conductive film 120, when seen fromthe above.

Alternatively, as illustrated in FIG. 31C, in a cross-sectional view inthe channel length direction, the channel region 108 a has a region thatdoes not overlap with the conductive film 120. The region functions asan offset region. That is, when seen from the above, the end portions ofthe low-resistance regions 108 b and 108 c do not overlap with the endportions of the conductive film 120.

As illustrated in FIG. 31D, in a cross-sectional view in the channellength direction, the low-resistance regions 108 b and 108 c each have aregion overlapping with the conductive film 120, specifically theconductive film 120 a. The region is referred to as an overlap region.That is, when seen from the above, the end portions of thelow-resistance regions 108 b and 108 c overlap with the conductive film120 a.

Alternatively, as illustrated in FIG. 31E, in a cross-sectional view inthe channel length direction, the low-resistance region 108 f betweenthe channel region 108 a and the low-resistance region 108 b, and thelow-resistance region 108 g between the channel region 108 a and thelow-resistance region 108 c are provided. The impurity element is addedto the low-resistance regions 108 f and 108 g through the conductivefilm 120 a; thus, the low-resistance regions 108 f and 108 g have lowerconcentrations of an impurity element and higher resistivity than thelow-resistance regions 108 b and 108 c. Here, the low-resistance regions108 f and 108 g overlap with the conductive film 120 a, but they mayoverlap with the conductive film 120 a and the conductive film 120 b.

As illustrated in FIG. 32A, in the cross-sectional view in the channellength direction, the end portion of the conductive film 120 a may bepositioned on an outer side than the end portion of the conductive film120 b and the conductive film 120 a may have a tapered shape. That is,an angle between a surface where the insulating film 117 and theconductive film 120 a are in contact with each other and a side surfaceof the conductive film 120 a may be less than 90°, greater than or equalto 5° and less than or equal to 45°, or greater than or equal to 5° andless than or equal to 30°.

Furthermore, the end portion of the insulating film 117 may bepositioned on an outer side than the end portion of the conductive film120 a.

Furthermore, a side surface of the insulating film 117 may be curved.

The insulating film 117 may have a tapered shape. That is, an angleformed between a surface where the oxide semiconductor film 108 and theinsulating film 117 are in contact with each other and a side surface ofthe insulating film 117 may be less than 90°, preferably greater than orequal to 30° and less than 90°.

The oxide semiconductor film 108 illustrated in FIG. 32A includes thechannel region 108 a, the low-resistance regions 108 f and 108 g betweenwhich the channel region 108 a is provided, low-resistance regions 108 hand 108 i between which the low-resistance regions 108 f and 108 g areprovided, and the low-resistance regions 108 b and 108 c between whichthe low-resistance regions 108 h and 108 i are provided. The impurityelement is added to the low-resistance regions 108 f, 108 g, 108 h, and108 i through the insulating film 117 and the conductive film 120 a;thus, the low-resistance regions 108 f, 108 g, 108 h, and 108 i havelower concentrations of an impurity element and higher resistivity thanthe low-resistance regions 108 b and 108 c.

The oxide semiconductor film 108 illustrated in FIG. 32B includes thechannel region 108 a, the low-resistance regions 108 h and 108 i betweenwhich the channel region 108 a is provided, and the low-resistanceregions 108 b and 108 c between which the low-resistance regions 108 hand 108 i are provided. The impurity element is added to thelow-resistance regions 108 h and 108 i through the insulating film 117;thus, the low-resistance regions 108 h and 108 i have lowerconcentrations of an impurity element and higher resistivity than thelow-resistance regions 108 b and 108 c.

Note that in the channel length direction, the channel region 108 aoverlaps with the conductive film 120 b, the low-resistance regions 108f and 108 g overlap with the conductive film 120 a projecting outsidethe conductive film 120 b, the low-resistance regions 108 h and 108 ioverlap with the insulating film 117 projecting outside the conductivefilm 120 a, and the low-resistance regions 108 b and 108 c arepositioned on outer sides than the insulating film 117.

As illustrated in FIG. 31E and FIGS. 32A and 32B, the oxidesemiconductor film 108 includes the low-resistance regions 108 f, 108 g,108 h, and 108 i having lower impurity element concentrations and higherresistivity than the low-resistance regions 108 b and 108 c, whereby theelectric field of the drain region can be relaxed. Thus, deteriorationof the transistor due to the electric field of the drain region, such asa shift of the threshold voltage of the transistor, can be inhibited.

The transistor shown in FIG. 33A includes the oxide semiconductor film108 including the channel region 108 a and the low-resistance regions108 b and 108 c. The low-resistance regions 108 b and 108 c each includea region with a thickness smaller than that of the channel region 108 a.Typically, the low-resistance regions 108 b and 108 c each include aregion with a thickness smaller than that of the channel region 108 a by0.1 nm or more and 5 nm or less.

In the transistor shown in FIG. 33B, at least one of the insulatingfilms 104 and 117, which are in contact with the oxide semiconductorfilm 108, has a multilayer structure. For example, the insulating film104 includes the insulating film 104 a and the insulating film 104 b incontact with the insulating film 104 a and the oxide semiconductor film108. For example, the insulating film 117 includes an insulating film117 a in contact with the oxide semiconductor film 108 and an insulatingfilm 117 b in contact with the insulating film 117 a.

The insulating films 104 b and 117 a can be formed using an oxideinsulating film with a low content of nitrogen oxide and a low densityof defect states. The oxide insulating film with a low content ofnitrogen oxide and a low density of defect states is, specifically, anoxide insulating film in which the density of defect states located 4.6eV or more and 8 eV or less lower than a vacuum level is low, that is,an oxide insulating film in which the density of defect statesattributed to nitrogen oxide is low. As the oxide insulating film with alow content of nitrogen oxide and a low density of defect states, asilicon oxynitride film that releases less nitrogen oxide, an aluminumoxynitride film that releases less nitrogen oxide, or the like can beused. The average thickness of each of the insulating films 104 b and117 a is greater than or equal to 0.1 nm and less than or equal to 50nm, or greater than or equal to 0.5 nm and less than or equal to 10 nm.

Note that a silicon oxynitride film that releases less nitrogen oxide isa film of which the amount of released ammonia is larger than the amountof released nitrogen oxide in thermal desorption spectroscopy (TDS)analysis; the amount of released ammonia is typically greater than orequal to 1× molecules/cm³ and less than or equal to 5×10¹⁹molecules/cm³. Note that the amount of released ammonia is the amount ofammonia released by heat treatment with which the surface temperature ofa film becomes higher than or equal to 50° C. and lower than or equal to650° C., preferably higher than or equal to 50° C. and lower than orequal to 550° C.

The insulating films 104 a and 117 b can be formed using an oxideinsulating film that releases oxygen by being heated. Note that theaverage thickness of each of the insulating films 104 a and 117 b isgreater than or equal to 5 nm and less than or equal to 1000 nm, orgreater than or equal to 10 nm and less than or equal to 500 nm.

Typical examples of the oxide insulating film that releases oxygen bybeing heated include a silicon oxynitride film and an aluminumoxynitride film.

Nitrogen oxide (NO_(x); x is greater than or equal to 0 and less than orequal to 2, preferably greater than or equal to 1 and less than or equalto 2), typically NO₂ or NO, forms levels in the insulating film 104, theinsulating film 117, and the like. The level is formed in the energy gapof the oxide semiconductor film 108. Therefore, when nitrogen oxide isdiffused to the interface between the insulating film 104 and the oxidesemiconductor film 108, the interface between the insulating film 117and the oxide semiconductor film 108, and the interface between theinsulating film 104 and the insulating film 117, an electron is trappedby the level on the insulating film 104 side and the insulating film 117side. As a result, the trapped electron remains in the vicinity of theinterface between the insulating film 104 and the oxide semiconductorfilm 108, the interface between the insulating film 117 and the oxidesemiconductor film 108, and the interface between the insulating film104 and the insulating film 117; thus, the threshold voltage of thetransistor is shifted in the positive direction.

Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Sincenitrogen oxide contained in the insulating films 104 a and 117 b reactswith ammonia contained in the insulating films 104 b and 117 a in heattreatment, nitrogen oxide contained in the insulating films 104 a and117 b is reduced. Therefore, an electron is hardly trapped at theinterface between the insulating film 104 and the oxide semiconductorfilm 108, the interface between the insulating film 117 and the oxidesemiconductor film 108, and the interface between the insulating film104 and the insulating film 117.

By using the oxide insulating film with a low content of nitrogen oxideand a low density of defect states for the insulating films 104 b and117 a, a shift in the threshold voltage of the transistor can bereduced, which leads to a smaller change in the electricalcharacteristics of the transistor.

Note that in an ESR spectrum at 100 K or lower of the insulating films104 b and 117 a, by heat treatment in a manufacturing process of thetransistor, typically heat treatment at a temperature higher than orequal to 300° C. and lower than the strain point of the substrate, afirst signal that appears at a g-factor of greater than or equal to2.037 and less than or equal to 2.039, a second signal that appears at ag-factor of greater than or equal to 2.001 and less than or equal to2.003, and a third signal that appears at a g-factor of greater than orequal to 1.964 and less than or equal to 1.966 are observed. The splitwidth of the first and second signals and the split width of the secondand third signals that are obtained by ESR measurement using an X-bandare each approximately 5 mT. The sum of the spin densities of the firstsignal that appears at a g-factor of greater than or equal to 2.037 andless than or equal to 2.039, the second signal that appears at ag-factor of greater than or equal to 2.001 and less than or equal to2.003, and the third signal that appears at a g-factor of greater thanor equal to 1.964 and less than or equal to 1.966 is lower than 1×10¹⁸spins/cm³, typically higher than or equal to 1×10¹⁷ spins/cm³ and lowerthan 1×10¹⁸ spins/cm³.

In the ESR spectrum at 100 K or lower, the first signal that appears ata g-factor of greater than or equal to 2.037 and less than or equal to2.039, the second signal that appears at a g-factor of greater than orequal to 2.001 and less than or equal to 2.003, and the third signalthat appears at a g-factor of greater than or equal to 1.964 and lessthan or equal to 1.966 correspond to signals attributed to nitrogendioxide (NO₂). In other words, the lower the total spin density of thefirst signal that appears at a g-factor of greater than or equal to2.037 and less than or equal to 2.039, the second signal that appears ata g-factor of greater than or equal to 2.001 and less than or equal to2.003, and the third signal that appears at a g-factor of greater thanor equal to 1.964 and less than or equal to 1.966 is, the lower thecontent of nitrogen oxide in the oxide insulating film is.

After heat treatment in a manufacturing process of the transistor,typically heat treatment at a temperature higher than or equal to 300°C. and lower than the strain point of the substrate, the oxideinsulating film with a low content of nitrogen oxide and a low densityof defect states has a nitrogen concentration measured by secondary ionmass spectrometry (SIMS) of 6×10²⁰ atoms/cm³ or lower.

By forming an oxide insulating film with a low content of nitrogen oxideand a low density of defect states by a plasma CVD method using silaneand dinitrogen monoxide at a substrate temperature higher than or equalto 220° C., higher than or equal to 280° C., or higher than or equal to350° C., a dense and hard film can be formed.

The transistor shown in FIG. 33C includes an insulating film 141 betweenthe insulating film 126 and the oxide semiconductor film 108, theinsulating film 117, and the conductive film 120. The insulating film141 can be formed using the oxide insulating film with a low content ofnitrogen oxide and a low density of defect states for the insulatingfilms 104 b and 117 a shown in FIG. 33B.

Alternatively, in a cross-sectional view in the channel lengthdirection, the low-resistance region 108 f between the channel region108 a and the low-resistance region 108 b, and the low-resistance region108 g between the channel region 108 a and the low-resistance region 108c are provided. The low-resistance regions 108 f and 108 g have lowerimpurity element concentrations and higher resistivity than thelow-resistance regions 108 b and 108 c. Here, the low-resistance regions108 f and 108 g overlap with the insulating film 141 in contact withside surfaces of the insulating film 117 and the conductive film 120.Note that the low-resistance regions 108 f and 108 g may overlap withthe insulating film 126 and the insulating film 141.

Note that in the transistor illustrated in FIG. 33D, the insulating film117 is in contact with the channel region 108 a of the oxidesemiconductor film 108 and is in contact with the low-resistance regions108 b and 108 c. Furthermore, in the insulating film 117, thicknesses ofregions in contact with the low-resistance regions 108 b and 108 c aresmaller than a thickness of a region in contact with the channel region108 a;

the average thickness of the insulating film 117 is typically greaterthan or equal to 0.1 nm and less than or equal to 50 nm, or greater thanor equal to 0.5 nm and less than or equal to 10 nm. As a result, theimpurity element can be added to the oxide semiconductor film 108through the insulating film 117, and in addition, hydrogen contained inthe insulating film 126 can be moved to the oxide semiconductor film 108through the insulating film 117. Thus, the low-resistance regions 108 band 108 c can be formed.

Furthermore, the insulating film 104 has a multilayer structure of theinsulating films 104 a and 104 b; for example, the insulating film 104 ais formed using an oxide insulating film that releases oxygen by beingheated, and the insulating film 104 b is formed using an oxideinsulating film with a low content of nitrogen oxide and a low densityof defect states. Furthermore, the insulating film 117 is formed usingan oxide insulating film with a low content of nitrogen oxide and a lowdensity of defect states. That is, the oxide semiconductor film 108 canbe covered with the oxide insulating film with a low content of nitrogenoxide and a low density of defect states. As a result, the carrier trapat the interfaces between the oxide semiconductor film 108 and theinsulating films 104 b and 117 can be reduced while oxygen contained inthe insulating film 104 a is moved to the oxide semiconductor film 108by heat treatment to reduce oxygen vacancies contained in the channelregion 108 a of the oxide semiconductor film 108. Consequently, a shiftin the threshold voltage of the transistor can be reduced, which leadsto a smaller change in the electrical characteristics of the transistor.

Embodiment 4

Here, a method in which a film which suppresses release of oxygen isformed over the insulating film and then oxygen is added to theinsulating film through the film is described with reference to FIGS.34A and 34B.

As shown in FIG. 34A, the insulating film 104 is formed over thesubstrate 101.

Next, a film 145 d which suppresses release of oxygen is formed over theinsulating film 104. Next, oxygen 146 d is added to the insulating film104 through the film 145 d.

The film 145 d which suppresses release of oxygen is formed using any ofthe following conductive materials: a metal element selected fromaluminum, chromium, tantalum, titanium, molybdenum, nickel, iron,cobalt, and tungsten; an alloy containing the above-described metalelement as a component; an alloy containing any of the above-describedmetal elements in combination; a metal nitride containing theabove-described metal element; a metal oxide containing theabove-described metal element; a metal nitride oxide containing theabove-described metal element; and the like.

The thickness of the film 145 d which suppresses release of oxygen canbe greater than or equal to 1 nm and less than or equal to 20 nm, orgreater than or equal to 2 nm and less than or equal to 10 nm.

As a method for adding the oxygen 146 d to the insulating film 104through the film 145 d, an ion doping method, an ion implantationmethod, plasma treatment, or the like is given. Note that it ispreferable that the film 145 d be exposed to plasma generated in a statewhere bias is applied to the substrate 101 side, because the amount ofoxygen added to the insulating film 104 can be increased. As an exampleof an apparatus used in such plasma treatment, an ashing apparatus isgiven.

By adding oxygen to the insulating film 104 with the film 145 d providedover the insulating film 104, the film 145 d serves as a protective filmwhich suppresses release of oxygen from the insulating film 104. Thus, alarger amount of oxygen can be added to the insulating film 104.

In the case where oxygen is added by plasma treatment, by making oxygenexcited by a microwave to generate high density oxygen plasma, theamount of oxygen added to the insulating film 104 can be increased.

After that, the film 145 d is removed; consequently, the insulating film104 to which oxygen is added can be formed over the substrate 101 asshown in FIG. 34B.

Embodiment 5

In this embodiment, V_(O)H which is formed in a low-resistance region ofan oxide semiconductor film is described.

<(1) Ease of Formation and Stability of V_(O)H>

In the case where an oxide semiconductor film (hereinafter referred toas IGZO) is a complete crystal, H preferentially diffuses along the a-bplane at a room temperature. In heat treatment at 450° C., H diffusesalong the a-b plane and in the c-axis direction. Here, calculation wasmade as to whether H easily enters an oxygen vacancy V_(O) if V_(O)exists in IGZO. A state in which 1-1 is in an oxygen vacancy V_(O) isreferred to as V_(O)H.

An InGaZnO₄ crystal model shown in FIG. 35 was used for calculation. Theactivation barrier (E_(a)) along the reaction path where H in V_(O)H isreleased from V_(O) and bonded to oxygen was calculated by a nudgedelastic band (NEB) method. The calculation conditions are shown in Table1.

TABLE 1 Software VASP Calculation method NEB method Functional GGA-PBEPseudopotential PAW Cut-off energy 500 eV K points 2 × 2 × 3

In the InGaZnO₄ crystal model, there are oxygen sites 1 to 4 as shown inFIG. 35 which differ from each other in metal elements bonded to oxygenand the number of bonded metal elements. Here, calculation was made onthe oxygen sites 1 and 2 in which an oxygen vacancy V_(O) is easilyformed.

First, calculation was made on the oxygen site in which an oxygenvacancy V_(O) is easily formed: an oxygen site 1 that was bonded tothree In atoms and one Zn atom.

FIG. 36A shows a model in the initial state and FIG. 36B shows a modelin the final state. FIG. 37 shows the calculated activation barrier(E_(a)) in the initial state and the final state. Note that here, theinitial state refers to a state in which H exists in an oxygen vacancyV_(O) (V_(O)H), and the final state refers to a structure including anoxygen vacancy V_(O) and a state in which H is bonded to oxygen bondedto one Ga atom and two Zn atoms (H—O).

From the calculation results, bonding of H in an oxygen vacancy V_(O) toanother oxygen atom needs an energy of approximately 1.52 eV, whileentry of H bonded to O into an oxygen vacancy V_(O) needs an energy ofapproximately 0.46 eV.

Reaction frequency (Γ) was calculated with use of the activationbarriers (E_(a)) obtained by the calculation and Formula 1. In Formula1, k_(B) represents the Boltzmann constant and T represents the absolutetemperature.

$\begin{matrix}{\Gamma = {v\mspace{14mu} {\exp \left( {- \frac{E_{a}}{k_{B}T}} \right)}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

The reaction frequency at 350° C. was calculated on the assumption thatthe frequency factor v=10¹³ [1/sec]. The frequency of H transfer fromthe model shown in FIG. 36A to the model shown in FIG. 36B was 5.52×10°[1/sec], whereas the frequency of H transfer from the model shown inFIG. 36B to the model shown in FIG. 36A was 1.82×10⁹ [1/sec]. Thissuggests that H diffusing in IGZO is likely to form V_(O)H if an oxygenvacancy V_(O) exists in the neighborhood, and H is unlikely to bereleased from the oxygen vacancy V_(O) once V_(O)H is formed.

Next, calculation was made on the oxygen site in which an oxygen vacancyV_(O) is easily formed: an oxygen site 2 that was bonded to one Ga atomand two Zn atoms.

FIG. 38A shows a model in the initial state and FIG. 38B shows a modelin the final state. FIG. 39 shows the calculated activation barrier(E_(a)) in the initial state and the final state. Note that here, theinitial state refers to a state in which H exists in an oxygen vacancyV_(O) (V_(O)H), and the final state refers to a structure including anoxygen vacancy V_(O) and a state in which H is bonded to oxygen bondedto one Ga atom and two Zn atoms (H—O).

From the calculation results, bonding of H in an oxygen vacancy V_(O) toanother oxygen atom needs an energy of approximately 1.75 eV, whileentry of H bonded to O into an oxygen vacancy V_(O) needs an energy ofapproximately 0.35 eV.

Reaction frequency (Γ) was calculated with use of the activationbarriers (E_(a)) obtained by the calculation and the above Formula 1.

The reaction frequency at 350° C. was calculated on the assumption thatthe frequency factor v=10¹³ [1/sec]. The frequency of H transfer fromthe model shown in FIG. 38A to the model shown in FIG. 38B was 7.53×10⁻²[1/sec], whereas the frequency of H transfer from the model shown inFIG. 38B to the model shown in FIG. 38A was 1.44×10¹⁰ [1/sec]. Thissuggests that H is unlikely to be released from the oxygen vacancy V_(O)once V_(O)H is formed.

From the above results, it was found that H in IGZO easily diffused inannealing and if an oxygen vacancy V_(O) existed, H was likely to enterthe oxygen vacancy V_(O) to be V_(O)H.

<(2) Transition Level of V_(O)H>

The calculation by the NEB method, which was described in <(1) Ease offormation and stability of V_(O)H>, indicates that in the case where anoxygen vacancy V_(O) and H exist in IGZO, the oxygen vacancy V_(O) and Heasily form V_(O)H and V_(O)H is stable. To determine whether V_(O)H isrelated to a carrier trap, the transition level of V_(O)H wascalculated.

The model used for calculation is an InGaZnO₄ crystal model (112 atoms).V_(O)H models of the oxygen sites 1 and 2 shown in FIG. 35 were made tocalculate the transition levels. The calculation conditions are shown inTable 2.

TABLE 2 Software VASP Model InGaZnO₄ crystal model (112 atoms)Functional HSE06 Mixture ratio of exchange terms 0.25 PseudopotentialGGA-PBE Cut-off energy 800 eV K points 1 × 1 × 1

The mixture ratio of exchange terms was adjusted to have a band gapclose to the experimental value. As a result, the band gap of theInGaZnO₄ crystal model without defects was 3.08 eV that is close to theexperimental value, 3.15 eV.

The transition level (ε(q/q′)) of a model having defect D can becalculated by the following Formula 2. Note that ΔE(D^(q)) representsthe formation energy of defect D at charge q, which is calculated byFormula 3.

$\begin{matrix}{\mspace{76mu} {{ɛ\left( {q\text{/}q^{\prime}} \right)} = \frac{{\Delta \; {E\left( D^{q} \right)}} - {\Delta \; {E\left( D^{q^{\prime}} \right)}}}{q^{\prime} - q}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack \\{{\Delta \; {E\left( D^{q} \right)}} = {{E_{tot}\left( D^{q} \right)} - {E_{tot}({bulk})} + {\sum\limits_{i}{\Delta \; n_{i}\mu_{i}}} + {q\left( {ɛ_{VBM} + {\Delta \; V_{q}} + E_{F}} \right)}}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack\end{matrix}$

In Formulae 2 and 3, E_(tot)(D^(q)) represents the total energy of themodel having defect D at the charge q in, E_(tot)(bulk) represents thetotal energy in a model without defects (complete crystal), Δn_(i)represents a change in the number of atoms i contributing to defects,μ_(i); represents the chemical potential of atom i, ε_(VBM) representsthe energy of the valence band maximum in the model without defects,Δv_(q) represents the correction term relating to the electrostaticpotential, and E_(F) represents the Fermi energy.

FIG. 40 shows the transition levels of V_(O)H obtained from the aboveformulae. The values in FIG. 40 represent the depth from the conductionband minimum. In FIG. 40, the transition level of V_(O)H in the oxygensite 1 is lower than the conduction band minimum by 0.05 eV, and thetransition level of V_(O)H in the oxygen site 2 is lower at 0.11 eV fromthe conduction band minimum. Therefore, these V_(O)H would be related toelectron traps, that is, V_(O)H was found to behave as a donor. It wasalso found that IGZO including V_(O)H had conductivity.

<Oxide Conductor Film>

The temperature dependence of the resistivity of an oxide conductor filmincluding V_(O)H is described with reference to FIG. 48.

In this embodiment, samples each including an oxide conductor film weremanufactured. As the oxide conductor film, an oxide conductor filmformed by making the oxide semiconductor film in contact with a siliconnitride film (OC_SiN_(x)), an oxide conductor film formed by making theoxide semiconductor film in contact with a silicon nitride film afteraddition of argon to the oxide semiconductor film with a dopingapparatus (OC_Ar dope+SiN_(x)), or an oxide conductor film formed bymaking the oxide semiconductor film in contact with a silicon nitridefilm after exposure of the oxide semiconductor film to argon plasma(OC_Ar plasma+SiN_(x)) with a plasma treatment apparatus was formed.Note that the silicon nitride film contains hydrogen.

A method for forming a sample including the oxide conductor film(OC_SiN_(x)) is as follows. A 400-nm-thick silicon oxynitride film wasformed over a glass substrate by a plasma CVD method and then exposed tooxygen plasma, and an oxygen ion was added to the silicon oxynitridefilm; accordingly, a silicon oxynitride film that releases oxygen byheating was formed. Next, a 100-nm-thick In—Ga—Zn oxide film was formedover the silicon oxynitride film that releases oxygen by heating by asputtering method using a sputtering target in which the atomic ratio ofIn to Ga and Zn was 1:1:1.2, and heat treatment was performed at 450° C.in a nitrogen atmosphere and then heat treatment was performed at 450°C. in a mixed atmosphere of nitrogen and oxygen. Then, a 100-nm-thicksilicon nitride film was formed by a plasma CVD method. After that, heattreatment was performed at 350° C. in a mixed gas atmosphere of nitrogenand oxygen.

A method for forming a sample including the oxide conductor film (OC_Ardope+SiN_(x)) is as follows. A 400-nm-thick silicon oxynitride film wasformed over a glass substrate by a plasma CVD method and then exposed tooxygen plasma, and an oxygen ion was added to the silicon oxynitridefilm; accordingly, a silicon oxynitride film that releases oxygen byheating was formed. Next, a 100-nm-thick In—Ga—Zn oxide film was formedover the silicon oxynitride film that releases oxygen by heating by asputtering method using a sputtering target in which the atomic ratio ofIn to Ga and Zn was 1:1:1.2, and heat treatment was performed at 450° C.in a nitrogen atmosphere and then heat treatment was performed at 450°C. in a mixed atmosphere of nitrogen and oxygen. Then, with a dopingapparatus, argon having a dose of 5×10¹⁴/cm² was added to the In—Ga—Znoxide film at an acceleration voltage of 10 kV, and oxygen vacancieswere formed in the In—Ga—Zn oxide film. After that, a 100-nm-thicksilicon nitride film was formed by a plasma CVD method. Subsequently,heat treatment was performed at 350° C. in a mixed gas atmosphere ofnitrogen and oxygen.

A method for forming a sample including the oxide conductor film (OC_Arplasma+SiN_(x)) is as follows. A 400-nm-thick silicon oxynitride filmwas formed over a glass substrate by a plasma CVD method and thenexposed to oxygen plasma; accordingly, a silicon oxynitride film thatreleases oxygen by heating was formed. Next, a 100-nm-thick In—Ga—Znoxide film was formed over the silicon oxynitride film that releasesoxygen by heating by a sputtering method using a sputtering target inwhich the atomic ratio of In to Ga and Zn was 1:1:1.2, and heattreatment was performed at 450° C. in a nitrogen atmosphere and thenheat treatment was performed at 450° C. in a mixed atmosphere ofnitrogen and oxygen. Then, in a plasma treatment apparatus, argon plasmawas generated, accelerated argon ions were made to collide with theIn—Ga—Zn oxide film, and oxygen vacancies were formed in the In—Ga—Znoxide film. After that, a 100-nm-thick silicon nitride film was formedby a plasma CVD method, Subsequently, heat treatment was performed at350° C. in a mixed gas atmosphere of nitrogen and oxygen.

Next, FIG. 48 shows the measured resistivity of the samples. Here, theresistivity was measured by the Van der Pauw method using fourterminals. In FIG. 48, the horizontal axis represents measurementtemperature, and the vertical axis represents resistivity. Measurementresults of the oxide conductor film (OC_SiN_(x)) are plotted as squares,measurement results of the oxide conductor film (OC_Ar dope+SiN_(x)) areplotted as circles, and measurement results of the oxide conductor film(OC_Ar plasma+SiN_(x)) are plotted as triangles.

Note that although not shown, the oxide semiconductor film which is notin contact with the silicon nitride film had high resistivity, which wasdifficult to measure. Therefore, it is found that the oxide conductorfilm has lower resistivity than the oxide semiconductor film.

According to FIG. 48, in the case where the oxide conductor film (OC_Ardope+SiN_(x)) and the oxide conductor film (OC_Ar plasma+SiN_(x))contain an oxygen vacancy and hydrogen, variation in resistivity issmall. Typically, the variation in resistivity at temperatures from 80 Kto 290 K is lower than ±20%. Alternatively, the variation in resistivityat temperatures from 150 K to 250 K is lower than ±10%. In other words,the oxide conductor is a degenerate semiconductor and it is suggestedthat the conduction band edge agrees with or substantially agrees withthe Fermi level. Thus, when the oxide conductor film is used as a sourceregion and a drain region of a transistor, an ohmic contact occurs at aportion where the oxide conductor film is in contact with conductivefilms functioning as a source electrode and a drain electrode, and thecontact resistance between the oxide conductor film and the conductivefilms functioning as a source electrode and a drain electrode can bereduced. Furthermore, the oxide conductor has low temperature resistanceof resistivity; thus, a fluctuation of contact resistance between theoxide conductor film and conductive films functioning as a sourceelectrode and a drain electrode is small, and a highly reliabletransistor can be obtained.

Embodiment 6

In this embodiment, the structure of an oxide semiconductor filmincluded in a semiconductor device of one embodiment of the presentinvention is described below in detail.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.The term “substantially parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −30° and lessthan or equal to 30°. The term “perpendicular” indicates that the angleformed between two straight lines is greater than or equal to 80° andless than or equal to 100°, and accordingly includes the case where theangle is greater than or equal to 85° and less than or equal to 95°. Theterm “substantially perpendicular” indicates that the angle formedbetween two straight lines is greater than or equal to 60° and less thanor equal to 120°.

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

<Structure of Oxide Semiconductor>

A structure of an oxide semiconductor is described below.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a CAAC-OS, apolycrystalline oxide semiconductor, a nanocrystalline oxidesemiconductor (nc-OS), an amorphous-like oxide semiconductor (a-likeOS), and an amorphous oxide semiconductor.

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor.Examples of a crystalline oxide semiconductor include a single crystaloxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor,and an nc-OS.

It is known that an amorphous structure is generally defined as beingmetastable and unfixed, and being isotropic and having no non-uniformstructure. In other words, an amorphous structure has a flexible bondangle and a short-range order but does not have a long-range order.

This means that an inherently stable oxide semiconductor cannot beregarded as a completely amorphous oxide semiconductor. Moreover, anoxide semiconductor that is not isotropic (e.g., an oxide semiconductorthat has a periodic structure in a microscopic region) cannot beregarded as a completely amorphous oxide semiconductor. Note that ana-like OS has a periodic structure in a microscopic region, but at thesame time has a void and has an unstable structure. For this reason, ana-like OS has physical properties similar to those of an amorphous oxidesemiconductor.

<CAAC-OSS

First, a CAAC-OS is described.

A CAAC-OS is one of oxide semiconductors having a plurality of c-axisaligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OS,which is obtained using a transmission electron microscope (TEM), aplurality of pellets can be observed. However, in the high-resolutionTEM image, a boundary between pellets, that is, a grain boundary is notclearly observed. Thus, in the CAAC-OS, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

A CAAC-OS observed with TEM is described below. FIG. 49A shows ahigh-resolution TEM image of a cross section of the CAAC-OS which isobserved from a direction substantially parallel to the sample surface.The high-resolution TEM image is obtained with a spherical aberrationcorrector function. The high-resolution TEM image obtained with aspherical aberration corrector function is particularly referred to as aCs-corrected high-resolution TEM image. The Cs-corrected high-resolutionTEM image can be obtained with, for example, an atomic resolutionanalytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 49B is an enlarged Cs-corrected high-resolution TEM image of aregion (1) in FIG. 49A. FIG. 49B shows that metal atoms are arranged ina layered manner in a pellet. Each metal atom layer has a configurationreflecting unevenness of a surface over which the CAAC-OS is formed(hereinafter, the surface is referred to as a formation surface) or atop surface of the CAAC-OS, and is arranged parallel to the formationsurface or the top surface of the CAAC-OS.

As shown in FIG. 49B, the CAAC-OS has a characteristic atomicarrangement. The characteristic atomic arrangement is denoted by anauxiliary line in FIG. 49C. FIGS. 49B and 49C prove that the size of apellet is approximately 1 nm to 3 nm, and the size of a space caused bytilt of the pellets is approximately 0.8 nm. Therefore, the pellet canalso be referred to as a nanocrystal (nc). Furthermore, the CAAC-OS canalso be referred to as an oxide semiconductor including c-axis alignednanocrystals (CANC).

Here, according to the Cs-corrected high-resolution TEM images, theschematic arrangement of pellets 5100 of a CAAC-OS over a substrate 5120is illustrated by such a structure in which bricks or blocks are stacked(see FIG. 49D). The part in which the pellets are tilted as observed inFIG. 49C corresponds to a region 5161 shown in FIG. 49D.

FIG. 50A shows a Cs-corrected high-resolution TEM image of a plane ofthe CAAC-OS observed from a direction substantially perpendicular to thesample surface. FIGS. 50B, 50C, and 50D are enlarged Cs-correctedhigh-resolution TEM images of regions (1), (2), and (3) in FIG. 50A,respectively. FIGS. 50B, 50C, and 50D indicate that metal atoms arearranged in a triangular, quadrangular, or hexagonal configuration in apellet. However, there is no regularity of arrangement of metal atomsbetween different pellets.

Next, a CAAC-OS analyzed by X-ray diffraction (XRD) is described. Forexample, when the structure of a CAAC-OS including an InGaZnO₄ crystalis analyzed by an out-of-plane method, a peak appears at a diffractionangle (2θ) of around 31° as shown in FIG. 51A. This peak is derived fromthe (009) plane of the InGaZnO₄ crystal, which indicates that crystalsin the CAAC-OS have c-axis alignment, and that the c-axes are aligned ina direction substantially perpendicular to the formation surface or thetop surface of the CAAC-OS.

Note that in structural analysis of the CAAC-OS by an out-of-planemethod, another peak may appear when 2θ is around 36°, in addition tothe peak at 2θ of around 31°. The peak at 2θ of around 36° indicatesthat a crystal having no c-axis alignment is included in part of theCAAC-OS. It is preferable that in the CAAC-OS analyzed by anout-of-plane method, a peak appear when 2θ is around 31° and that a peaknot appear when 2θ is around 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray beam is incident on a sample in a directionsubstantially perpendicular to the c-axis, a peak appears when 2θ isaround 56°. This peak is attributed to the (110) plane of the InGaZnO₄crystal. In the case of the CAAC-OS, when analysis (ϕ scan) is performedwith 2θ fixed at around 56° and with the sample rotated using a normalvector of the sample surface as an axis (ϕ axis), as shown in FIG. 51B,a peak is not clearly observed. In contrast, in the case of a singlecrystal oxide semiconductor of InGaZnO₄, when ϕ scan is performed with2θ fixed at around 56°, as shown in FIG. 51C, six peaks which arederived from crystal planes equivalent to the (110) plane are observed.Accordingly, the structural analysis using XRD shows that the directionsof a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the sample surface, a diffraction pattern (also referred toas a selected-area transmission electron diffraction pattern) shown inFIG. 52A can be obtained. In this diffraction pattern, spots derivedfrom the (009) plane of an InGaZnO₄ crystal are included. Thus, theelectron diffraction also indicates that pellets included in the CAAC-OShave c-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 52B shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 52B, a ring-like diffraction pattern isobserved. Thus, the electron diffraction also indicates that the a-axesand b-axes of the pellets included in the CAAC-OS do not have regularalignment. The first ring in FIG. 52B is considered to be derived fromthe (010) plane, the (100) plane, and the like of the InGaZnO₄ crystal.The second ring in FIG. 52B is considered to be derived from the (110)plane and the like.

As described above, the CAAC-OS is an oxide semiconductor with highcrystallinity. Entry of impurities, formation of defects, or the likemight decrease the crystallinity of an oxide semiconductor. This meansthat the CAAC-OS has small amounts of impurities and defects (e.g.,oxygen vacancies).

Note that the impurity means an element other than the main componentsof the oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (specifically, siliconor the like) having higher strength of bonding to oxygen than a metalelement included in an oxide semiconductor extracts oxygen from theoxide semiconductor, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor. A heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities ordefects might be changed by light, heat, or the like. Impuritiescontained in the oxide semiconductor might serve as carrier traps orcarrier generation sources, for example. Furthermore, oxygen vacanciesin the oxide semiconductor serve as carrier traps or serve as carriergeneration sources when hydrogen is captured therein.

The CAAC-OS having small amounts of impurities and oxygen vacancies isan oxide semiconductor with low carrier density (specifically, lowerthan 8×10¹¹/cm³, preferably lower than 1×10¹¹/cm³, further preferablylower than 1×10¹⁰/cm³, and is higher than or equal to 1×10⁻⁹/cm³). Suchan oxide semiconductor is referred to as a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. A CAAC-OShas a low impurity concentration and a low density of defect states.Thus, the CAAC-OS can be referred to as an oxide semiconductor havingstable characteristics.

<nc-OS>

Next, an nc-OS will be described.

An nc-OS has a region in which a crystal part is observed and a regionin which a crystal part is not clearly observed in a high-resolution TEMimage. In most cases, the size of a crystal part included in the nc-OSis greater than or equal to 1 nm and less than or equal to 10 nm, orgreater than or equal to 1 nm and less than or equal to 3 nm. Note thatan oxide semiconductor including a crystal part whose size is greaterthan 10 nm and less than or equal to 100 nm is sometimes referred to asa microcrystalline oxide semiconductor. In a high-resolution TEM imageof the nc-OS, for example, a grain boundary is not clearly observed insome cases. Note that there is a possibility that the origin of thenanocrystal is the same as that of a pellet in a CAAC-OS. Therefore, acrystal part of the nc-OS may be referred to as a pellet in thefollowing description.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 μm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different pellets in thenc-OS. Thus, the orientation of the whole film is not ordered.Accordingly, the nc-OS cannot be distinguished from an a-like OS or anamorphous oxide semiconductor, depending on an analysis method. Forexample, when the nc-OS is analyzed by an out-of-plane method using anX-ray beam having a diameter larger than the size of a pellet, a peakwhich shows a crystal plane does not appear. Furthermore, a diffractionpattern like a halo pattern is observed when the nc-OS is subjected toelectron diffraction using an electron beam with a probe diameter (e.g.,50 nm or larger) that is larger than the size of a pellet. Meanwhile,spots appear in a nanobeam electron diffraction pattern of the nc-OSwhen an electron beam having a probe diameter close to or smaller thanthe size of a pellet is applied. Moreover, in a nanobeam electrondiffraction pattern of the nc-OS, regions with high luminance in acircular (ring) pattern are shown in some cases. Also in a nanobeamelectron diffraction pattern of the nc-OS, a plurality of spots is shownin a ring-like region in some cases.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including random aligned nanocrystals (RANC) oran oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as comparedwith an amorphous oxide semiconductor. Therefore, the nc-OS is likely tohave a lower density of defect states than an a-like OS and an amorphousoxide semiconductor. Note that there is no regularity of crystalorientation between different pellets in the nc-OS. Therefore, the nc-OShas a higher density of defect states than the CAAC-OS.

<A-Like OS>

An a-like OS has a structure intermediate between those of the nc-OS andthe amorphous oxide semiconductor.

In a high-resolution TEM image of the a-like OS, a void may be observed.Furthermore, in the high-resolution TEM image, there are a region wherea crystal part is clearly observed and a region where a crystal part isnot observed.

The a-like OS has an unstable structure because it includes a void. Toverify that an a-like OS has an unstable structure as compared with aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation is described below.

An a-like OS (referred to as Sample A), an nc-OS (referred to as SampleB), and a CAAC-OS (referred to as Sample C) are prepared as samplessubjected to electron irradiation. Each of the samples is an In—Ga—Znoxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

Note that which part is regarded as a crystal part is determined asfollows. It is known that a unit cell of an InGaZnO₄ crystal has astructure in which nine layers including three In—O layers and sixGa—Zn—O layers are stacked in the c-axis direction. The distance betweenthe adjacent layers is equivalent to the lattice spacing on the (009)plane (also referred to as d value). The value is calculated to be 0.29nm from crystal structural analysis. Accordingly, a portion where thelattice spacing between lattice fringes is greater than or equal to 0.28nm and less than or equal to 0.30 nm is regarded as a crystal part ofInGaZnO₄. Each of lattice fringes corresponds to the a-b plane of theInGaZnO₄ crystal.

FIG. 53 shows change in the average size of crystal parts (at 22 pointsto 45 points) in each sample, Note that the crystal part sizecorresponds to the length of a lattice fringe. FIG. 53 indicates thatthe crystal part size in the a-like OS increases with an increase in thecumulative electron dose. Specifically, as shown by (1) in FIG, 53, acrystal part of approximately 1.2 nm (also referred to as an initialnucleus) at the start of TEM observation grows to a size ofapproximately 2.6 nm at a cumulative electron dose of 4.2×10⁸ e⁻/nm². Incontrast, the crystal part size in the nc-OS and the CAAC-OS showslittle change from the start of electron irradiation to a cumulativeelectron dose of 4.2×10⁸ e⁻/nm². Specifically, as shown by (2) and (3)in FIG. 53, the average crystal sizes in an nc-OS and a CAAC-OS areapproximately 1.4 nm and approximately 2.1 nm, respectively, regardlessof the cumulative electron dose.

In this manner, growth of the crystal part in the a-like OS is inducedby electron irradiation. In contrast, in the nc-OS and the CAAC-OS,growth of the crystal part is hardly induced by electron irradiation.Therefore, the a-like OS has an unstable structure as compared with thenc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit includes a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of the singlecrystal oxide semiconductor having the same composition. The density ofeach of the nc-OS and the CAAC-OS is higher than or equal to 92.3% andlower than 100% of the density of the single crystal oxide semiconductorhaving the same composition. Note that it is difficult to deposit anoxide semiconductor having a density of lower than 78% of the density ofthe single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomicratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, thedensity of the a-like OS is higher than or equal to 5.0 g/cm³ and lowerthan 5.9 g/cm³. For example, in the case of the oxide semiconductorhaving an atomic ratio of In:Ga:Zn=1:1:1, the density of each of thenc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

Note that there is a possibility that an oxide semiconductor having acertain composition cannot exist in a single crystal structure. In thatcase, single crystal oxide semiconductors with different compositionsare combined at an adequate ratio, which makes it possible to calculatedensity equivalent to that of a single crystal oxide semiconductor withthe desired composition. The density of a single crystal oxidesemiconductor having the desired composition can be calculated using aweighted average according to the combination ratio of the singlecrystal oxide semiconductors with different compositions. Note that itis preferable to use as few kinds of single crystal oxide semiconductorsas possible to calculate the density.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a stackedlayer including two or more of an amorphous oxide semiconductor, ana-like OS, an nc-OS, and a CAAC-OS, for example.

The structure and method described in this embodiment can be combined asappropriate with any of the other structures and methods described inthe other embodiments.

Embodiment 7

In this embodiment, a display device that can be formed using asemiconductor device of one embodiment of the present invention isdescribed with reference to FIGS. 41A to 41D.

The display device illustrated in FIG. 41A includes a region includingpixels of display elements (hereinafter the region is referred to as apixel portion 542), a circuit portion being provided outside the pixelportion 542 and including a circuit for driving the pixels (hereinafterthe portion is referred to as a driver circuit portion 544), circuitseach having a function of protecting an element (hereinafter thecircuits are referred to as protection circuits 546), and a terminalportion 547. Note that the protection circuits 546 are not necessarilyprovided.

A part or the whole of the driver circuit portion 544 is preferablyformed over a substrate over which the pixel portion 542 is formed, inwhich case the number of components and the number of terminals can bereduced. When a part or the whole of the driver circuit portion 544 isnot formed over the substrate over which the pixel portion 542 isformed, the part or the whole of the driver circuit portion 544 can bemounted by chip on glass (COG) or tape automated bonding (TAB).

The pixel portion 542 includes a plurality of circuits for drivingdisplay elements arranged in X rows (X is a natural number of 2 or more)and Y columns (Y is a natural number of 2 or more) (hereinafter, suchcircuits are referred to as pixel circuits 541). The driver circuitportion 544 includes driver circuits such as a circuit for supplying asignal (scan signal) to select a pixel (hereinafter, the circuit isreferred to as a gate driver 544 a) and a circuit for supplying a signal(data signal) to drive a display element in a pixel (hereinafter, thecircuit is referred to as a source driver 544 b).

The gate driver 544 a includes a shift register or the like. The gatedriver 544 a receives a signal for driving the shift register throughthe terminal portion 547 and outputs a signal. For example, the gatedriver 544 a receives a start pulse signal, a clock signal, or the likeand outputs a pulse signal. The gate driver 544 a has a function ofcontrolling the potentials of wirings supplied with scan signals(hereinafter, such wirings are referred to as scan lines GL_1 to GL_X).Note that a plurality of gate drivers 544 a may be provided to controlthe scan lines GL_1 to GL_X separately. Alternatively, the gate driver544 a has a function of supplying an initialization signal. Withoutbeing limited thereto, the gate driver 544 a can supply another signal.

The source driver 544 b includes a shift register or the like. Thesource driver 544 b receives a signal (video signal) from which a datasignal is derived, as well as a signal for driving the shift register,through the terminal portion 547. The source driver 544 b has a functionof generating a data signal to be written to the pixel circuit 541 whichis based on the video signal. In addition, the source driver 544 b has afunction of controlling output of a data signal in response to a pulsesignal produced by input of a start pulse signal, a clock signal, or thelike. Furthermore, the source driver 544 b has a function of controllingthe potentials of wirings supplied with data signals (hereinafter suchwirings are referred to as signal lines DL_1 to DL_Y). Alternatively,the source driver 544 b has a function of supplying an initializationsignal. Without being limited thereto, the source driver 544 b cansupply another signal.

The source driver 544 b includes a plurality of analog switches or thelike, for example. The source driver 544 b can output, as the datasignals, signals obtained by time-dividing the video signal bysequentially turning on the plurality of analog switches. The sourcedriver 544 b may include a shift register or the like.

A pulse signal and a data signal are input to each of the plurality ofpixel circuits 541 through one of the plurality of scan lines GLsupplied with scan signals and one of the plurality of signal lines DLsupplied with data signals, respectively. Writing and holding of thedata signal to and in each of the plurality of pixel circuits 541 arecontrolled by the gate driver 544 a. For example, to the pixel circuit541 in the m-th row and the n-th column (m is a natural number of lessthan or equal to A′, and n is a natural number of less than or equal toY), a pulse signal is input from the gate driver 544 a through the scanline GL_m, and a data signal is input from the source driver 544 bthrough the signal line DL_n in accordance with the potential of thescan line GL_m.

The protection circuit 546 shown in FIG. 41A is connected to, forexample, the scan line GL between the gate driver 544 a and the pixelcircuit 541. Alternatively, the protection circuit 546 is connected tothe signal line DL between the source driver 544 b and the pixel circuit541. Alternatively, the protection circuit 546 can be connected to awiring between the gate driver 544 a and the terminal portion 547.Alternatively, the protection circuit 546 can be connected to a wiringbetween the source driver 544 b and the terminal portion 547. Note thatthe terminal portion 547 means a portion having terminals for inputtingpower, control signals, and video signals to the display device fromexternal circuits.

The protection circuit 546 is a circuit that electrically connects awiring connected to the protection circuit to another wiring when apotential out of a certain range is applied to the wiring connected tothe protection circuit.

As illustrated in FIG. 41A, the protection circuits 546 are provided forthe pixel portion 542 and the driver circuit portion 544, so that theresistance of the display device to overcurrent generated byelectrostatic discharge (ESD) or the like can be improved. Note that theconfiguration of the protection circuits 546 is not limited to that, andfor example, the protection circuit 546 may be configured to beconnected to the gate driver 544 a or the protection circuit 546 may beconfigured to be connected to the source driver 544 b. Alternatively,the protection circuit 546 may be configured to be connected to theterminal portion 547.

In FIG. 41A, an example in which the driver circuit portion 544 includesthe gate driver 544 a and the source driver 544 b is shown; however, thestructure is not limited thereto. For example, only the gate driver 544a may be formed and a separately prepared substrate where a sourcedriver circuit is formed (e.g., a driver circuit substrate formed with asingle crystal semiconductor film or a polycrystalline semiconductorfilm) may be mounted.

Each of the plurality of pixel circuits 541 in FIG. 41A can have thestructure illustrated in FIG. 41B, for example.

The pixel circuit 541 illustrated in FIG. 41B includes a liquid crystalelement 570, a transistor 550, and a capacitor 560.

As the transistor 550, any of the transistors described in the aboveembodiments, for example, can be used as appropriate.

The potential of one of a pair of electrodes of the liquid crystalelement 570 is set in accordance with the specifications of the pixelcircuit 541 as appropriate. The alignment state of the liquid crystalelement 570 depends on written data. A common potential may be suppliedto one of the pair of electrodes of the liquid crystal element 570included in each of the plurality of pixel circuits 541, Furthermore,the potential supplied to one of the pair of electrodes of the liquidcrystal element 570 in the pixel circuit 541 in one row may be differentfrom the potential supplied to one of the pair of electrodes of theliquid crystal element 570 in the pixel circuit 541 in another row.

In the pixel circuit 541 in the m-th row and the n-th column, one of asource electrode and a drain electrode of the transistor 550 iselectrically connected to the signal line DL_n, and the other iselectrically connected to the other of the pair of electrodes of theliquid crystal element 570. A gate electrode of the transistor 550 iselectrically connected to the scan line GL_m. The transistor 550 has afunction of controlling whether to write a data signal by being turnedon or off.

One of a pair of electrodes of the capacitor 560 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as a potential supply line VL), and the other iselectrically connected to the other of the pair of electrodes of theliquid crystal element 570. The potential of the potential supply lineVL is set in accordance with the specifications of the pixel circuit 541as appropriate. The capacitor 560 functions as a storage capacitor forstoring written data.

For example, in the display device including the pixel circuit 541 inFIG. 41B, the pixel circuits 541 are sequentially selected row by row bythe gate driver 544 a illustrated in FIG. 41A, whereby the transistors550 are turned on and a data signal is written.

When the transistors 550 are turned off, the pixel circuits 541 in whichthe data has been written are brought into a holding state. Thisoperation is sequentially performed row by row; thus, an image can bedisplayed.

Alternatively, each of the plurality of pixel circuits 541 in FIG. 41Acan have the structure illustrated in FIG. 41C, for example.

The pixel circuit 541 illustrated in FIG. 41C includes transistors 552and 554, a capacitor 562, and a light-emitting element 572. Here, any ofthe transistors described in the above embodiments, for example, can beused as one or both of the transistors 552 and 554 as appropriate.

One of a source electrode and a drain electrode of the transistor 552 iselectrically connected to a wiring to which a data signal is supplied (asignal line DL_n). A gate electrode of the transistor 552 iselectrically connected to a wiring to which a gate signal is supplied (ascan line GL_m).

The transistor 552 has a function of controlling whether to write a datasignal by being turned on or off.

A transistor having positive threshold voltage (normally-offcharacteristics) is preferably used as the transistor 552. The cutoffcurrent of the transistor is preferably reduced. Accordingly, it ispreferable to use any of the transistors 100 h, 100 j, 100 z, 100 n, and100 y described in Embodiment 1 and the transistors 100 t, 100 w, 111 b,111 e, 111 h, and 111 k described in Embodiment 2 as appropriate.

One of a pair of electrodes of the capacitor 562 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as a potential supply line VL_a), and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 552.

The capacitor 562 functions as a storage capacitor for storing writtendata.

One of a source electrode and a drain electrode of the transistor 554 iselectrically connected to the potential supply line VL_a. Furthermore, agate electrode of the transistor 554 is electrically connected to theother of the source electrode and the drain electrode of the transistor552.

The transistor 554 has a function of controlling current flowing in thelight-emitting element 572 by being turned on or off.

To obtain sufficient luminance for the light-emitting element 572, atransistor having high on-state current needs to be used as thetransistor 554 serving as a driver transistor. Further, to improve thedrive frequency of the display device and achieve smoother imagedisplay, the transistor needs to have high field-effect mobility.Accordingly, it is preferable to use, as the transistor 554, any of thetransistors 100 u, 100 x, 111 c, 111 f, 111 i, and 111 m described inEmbodiment 2 as appropriate.

A transistor with a short channel length can have high field-effectmobility; at the same time, the threshold voltage of the transistormight shift in the negative direction in some cases. The channel lengthof the transistor 554 is set to greater than or equal to 0.5 μm and lessthan or equal to 4.5 μm and a pair of gate electrodes electricallyconnected to each other is provided as in the transistors 111 i and 111m described in Embodiment 2; thus, the on-state current and thefield-effect mobility can be improved and a negative shift in thethreshold voltage can be suppressed.

In contrast, because the transistor 552 serving as a selectiontransistor does not require field-effect mobility as high as that of thetransistor 554, the channel length of the transistor 552 is made longerthan that of the transistor 554. As a result, a negative shift in thethreshold voltage of the transistor 554 is suppressed. With such astructure, the display device can operate at high speed with low powerconsumption.

In the case where the channel length of the transistor 554 is set togreater than or equal to 0.5 μm and less than or equal to 4.5 μm, forexample, the channel length of the transistor 552 can be 6 μm. Note thatthe channel length of the transistor 552 can be set as appropriatedepending on characteristics required for the display device, as long asit is at least longer than the channel length of the transistor 554.

Further, the cutoff current of the transistor 552 is preferably lowerthan that of the transistor 554. For example, when the ratio of thechannel length to the channel width (L/W ratio) of the transistor 552 ismade larger than the L/W ratio of the transistor 554, the cutoff currentof the transistor 552 can be lower than that of the transistor 554. Inthe case where the channel widths of the transistor 554 and thetransistor 552 are substantially the same, the channel length of thetransistor 552 is made longer than that of the transistor 554, wherebythe cutoff current of the transistor 552 can be reduced.

The transistor 552 may include a pair of gate electrodes electricallyconnected to each other. Note that when the transistor 552 has asingle-gate structure, a region where the pair of gate electrodes areconnected to each other can be omitted, so that the area of thetransistor can be reduced and a decrease in the aperture ratio of thepixel can be prevented. In the case where the display device isincreased in size, parasitic capacitance of a gate wiring which isapplied to the transistor 552 serving as a selection transistor of thepixel is increased; accordingly, using the transistor 552 with asingle-gate structure is effective.

One of an anode and a cathode of the light-emitting element 572 iselectrically connected to a potential supply line VL_b, and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 554.

As the light-emitting element 572, an organic electroluminescent element(also referred to as an organic EL element) or the like can be used, forexample. Note that the light-emitting element 572 is not limited to anorganic EL element; an inorganic EL element including an inorganicmaterial may be used.

A high power supply potential VDD is supplied to one of the potentialsupply line VL_a and the potential supply line VL_b, and a low powersupply potential VSS is supplied to the other.

For example, in the display device including the pixel circuit 541 inFIG. 41C, the pixel circuits 541 are sequentially selected row by row bythe gate driver 544 a illustrated in FIG. 41A, whereby the transistors552 are turned on and a data signal is written.

When the transistors 552 are turned off, the pixel circuits 541 in whichthe data has been written are brought into a holding state. Furthermore,the amount of current flowing between the source electrode and the drainelectrode of the transistor 554 is controlled in accordance with thepotential of the written data signal. The light-emitting element 572emits light with a luminance corresponding to the amount of flowingcurrent. This operation is sequentially performed row by row; thus, animage can be displayed.

Alternatively, each of the plurality of pixel circuits 541 in FIG. 41Acan have the structure illustrated in FIG. 41D, for example.

The pixel circuit 541 illustrated in FIG. 41D includes the transistor552 serving as a selection transistor which controls writing of a datasignal, the transistor 554 serving as a driver transistor, a transistor556, the capacitor 562, and the light-emitting element 572. Here, any ofthe transistors 100 t, 100 w, 111 b, 111 e, 111 h, and 111 k describedin Embodiment 2 can be used as the transistor 552 as appropriate, andany of the transistors 100 u, 100 x, 111 c, 111 f, 111 i, and 111 mdescribed in Embodiment 2 can be used as the transistor 554 asappropriate.

One of a source electrode and a drain electrode of the transistor 552 iselectrically connected to a wiring to which a data signal is supplied (asignal line DL_n). A gate electrode of the transistor 552 iselectrically connected to a wiring to which a gate signal is supplied (ascan line GL_m).

The transistor 552 has a function of controlling whether to write a datasignal by being turned on or off. This means that the transistor 552serves as a selection transistor.

One of a source electrode and a drain electrode of the transistor 554 iselectrically connected to a wiring to which a potential is supplied(hereinafter referred to as a potential supply line VL_a1), and theother of the source electrode and the drain electrode of the transistor554 is electrically connected to one electrode of the light-emittingelement 572. A gate electrode of the transistor 554 is electricallyconnected to the other of the source electrode and the drain electrodeof the transistor 552 and one electrode of the capacitor 562.

One of a source electrode and a drain electrode of the transistor 556 isconnected to a wiring ML to which a reference potential of data issupplied, and the other is electrically connected to the one electrodeof the light-emitting element 572 and the other electrode of thecapacitor 562. A gate electrode of the transistor 556 is electricallyconnected to the scan line GL_m to which a gate signal is supplied.

The transistor 556 has a function of controlling current flowing throughthe light-emitting element 572. For example, in the case where thetransistors 554 deteriorate or vary in threshold voltage or field-effectmobility, current that flows in each light-emitting element 572 can becorrected through monitoring of current flowing in the wiring MLconnected to one of the source electrode and the drain electrode of eachtransistor 556. The wiring ML can be supplied with, for example, voltagelower than or equal to the threshold voltage of the light-emittingelement 572.

In this embodiment, the channel length of the transistor 556 ispreferably longer than the channel length of the transistor 554, forexample. Note that the transistor 556 can have a single-gate structure,or have a dual-gate structure like the transistor 554. It is preferablethat the transistor 556 have a single-gate structure because a regionwhere a first gate electrode and a second gate electrode are in contactwith each other is not necessary, so that the area of the transistor canbe reduced, which can increase the aperture ratio of the pixel.

The one of the pair of electrodes of the capacitor 562 is electricallyconnected to the other of the source electrode and the drain electrodeof the transistor 552 and the gate electrode of the transistor 554. Theother of the pair of electrodes of the capacitor 562 is electricallyconnected to the other of the source electrode and the drain electrodeof the transistor 554, the other of the source electrode and the drainelectrode of the transistor 556, and the one electrode of thelight-emitting element 572.

The one of the pair of electrodes of the light-emitting element 572 iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 554, the other electrode of thecapacitor 562, and the other of the source electrode and the drainelectrode of the transistor 556. The other of the pair of electrodes ofthe light-emitting element 572 is electrically connected to a potentialsupply line VL_b serving as a cathode.

A potential supply line VL_a2 extends in a direction parallel to thewiring ML. The potential supply line VL_a2 is connected to the potentialsupply line VL_a1 serving as an anode line, so that the wiringresistance of the potential supply lines VL_a1 and VL_a2 can be reduced.Thus, voltage drop of the wiring in the display device using a largesubstrate can be reduced; accordingly, unevenness in luminance of thedisplay device can be reduced.

One of a high power supply potential VDD and a low power supply VSS issupplied to the potential supply lines VL_a1 and VL_a2, and the other ofthe high power supply potential VDD and the low power supply potentialVSS is supplied to the potential supply line VL_b. In the structure ofFIG. 41D, the high power supply potential VDD is supplied to thepotential supply lines VL_a1 and VL_a2, and the low power supplypotential VSS is supplied to the potential supply line VL_b.

In the display device including the pixel circuit 541 in FIG. 41D, thepixel circuits 541 are sequentially selected row by row by the gatedriver 544 a illustrated in FIG. 41A, for example, whereby thetransistors 552 are turned on and a data signal is written.

When the transistors 552 are turned off, the pixel circuits 541 in whichthe data has been written are brought into a holding state. Moreover,the transistor 552 is connected to the capacitor 562; thus, the writtendata can be held for a long time. Further, the amount of current flowingbetween the source electrode and the drain electrode of the transistor554 is controlled. The light-emitting element 572 emits light with aluminance corresponding to the amount of flowing current. This operationis sequentially performed row by row; thus, an image can be displayed.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 8

In this embodiment, an example of a display device that includes any ofthe transistors described in the above embodiments is described belowwith reference to FIG. 42, FIGS. 43A and 43B, and FIGS. 44A and 44B.

FIG. 42 is a top view of an example of a display device. A displaydevice 700 illustrated in FIG. 42 includes a pixel portion 702 providedover a first substrate 701; a source driver circuit portion 704 and agate driver circuit portion 706 provided over the first substrate 701; asealant 712 provided to surround the pixel portion 702, the sourcedriver circuit portion 704, and the gate driver circuit portion 706; anda second substrate 705 provided to face the first substrate 701. Thefirst substrate 701 and the second substrate 705 are sealed with thesealant 712. That is, the pixel portion 702, the source driver circuitportion 704, and the gate driver circuit portion 706 are sealed with thefirst substrate 701, the sealant 712, and the second substrate 705.Although not illustrated in FIG. 42, a display element is providedbetween the first substrate 701 and the second substrate 705.

In the display device 700, a flexible printed circuit (FPC) terminalportion 708 electrically connected to the pixel portion 702, the sourcedriver circuit portion 704, and the gate driver circuit portion 706 isprovided in a region different from the region which is surrounded bythe sealant 712 and positioned over the first substrate 701.Furthermore, an FPC 716 is connected to the FPC terminal portion 708,and a variety of signals and the like are supplied to the pixel portion702, the source driver circuit portion 704, and the gate driver circuitportion 706 through the FPC 716. Furthermore, a signal line 710 isconnected to the pixel portion 702, the source driver circuit portion704, the gate driver circuit portion 706, and the FPC terminal portion708. Various signals and the like are applied to the pixel portion 702,the source driver circuit portion 704, the gate driver circuit portion706, and the FPC terminal portion 708 via the signal line 710 from theFPC 716.

A plurality of gate driver circuit portions 706 may be provided in thedisplay device 700. An example of the display device 700 in which thesource driver circuit portion 704 and the gate driver circuit portion706 are fainted over the first substrate 701 where the pixel portion 702is also formed is described; however, the structure is not limitedthereto. For example, only the gate driver circuit portion 706 may beformed over the first substrate 701 or only the source driver circuitportion 704 may be formed over the first substrate 701. In this case, asubstrate where a source driver circuit, a gate driver circuit, or thelike is formed (e.g., a driver-circuit substrate formed using asingle-crystal semiconductor film or a polycrystalline semiconductorfilm) may be mounted on the first substrate 701. Note that there is noparticular limitation on the method of connecting a separately prepareddriver circuit substrate, and a COG method, a wire bonding method, orthe like can be used.

The pixel portion 702, the source driver circuit portion 704, and thegate driver circuit portion 706 included in the display device 700include a plurality of transistors. As the plurality of transistors, anyof the transistors that are the semiconductor devices of embodiments ofthe present invention can be used.

The display device 700 can include any of a variety of elements.Examples of the element include a liquid crystal element, anelectroluminescence (EL) element (e.g., an EL element including organicand inorganic materials, an organic EL element, or an inorganic ELelement), an LED (e.g., a white LED, a red LED, a green LED, or a blueLED), a transistor (a transistor that emits light depending on current),an electron emitter, electronic ink, an electrophoretic element, agrating light valve (GLV), a plasma display panel (PDP), a displayelement using micro electro mechanical system (MEMS), a digitalmicromirror device (DMD), a digital micro shutter (DMS), MIRASOL(registered trademark), an interferometric modulator display (IMOD)element, a MEMS shutter display element, an optical-interference-typeMEMS display element, an electrowetting element, a piezoelectric ceramicdisplay, and a display element including a carbon nanotube. Other thanthe above, display media whose contrast, luminance, reflectivity,transmittance, or the like is changed by electrical or magnetic effectmay be included. Note that examples of display devices having ELelements include an EL display. Examples of display devices includingelectron emitters include a field emission display (FED) and an SED-typeflat panel display (SED: surface-conduction electron-emitter display).Examples of display devices including liquid crystal elements include aliquid crystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display). An example of a display device including electronicink or electrophoretic elements is electronic paper. In the case of atransflective liquid crystal display or a reflective liquid crystaldisplay, some of or all of pixel electrodes function as reflectiveelectrodes. For example, some or all of pixel electrodes are formed tocontain aluminum, silver, or the like. In such a case, a memory circuitsuch as an SRAM can be provided under the reflective electrodes, leadingto lower power consumption.

As a display method in the display device 700, a progressive method, aninterlace method, or the like can be employed. Furthermore, colorelements controlled in a pixel at the time of color display are notlimited to three colors: R, G, and B (R, G, and B correspond to red,green, and blue, respectively). For example, four pixels of the R pixel,the G pixel, the B pixel, and a W (white) pixel may be included.Alternatively, a color element may be composed of two colors among R, G,and B as in PenTile layout. The two colors may differ among colorelements. Alternatively, one or more colors of yellow, cyan, magenta,and the like may be added to RGB. Further, the size of a display regionmay be different depending on respective dots of the color components.Embodiments of the disclosed invention are not limited to a displaydevice for color display; the disclosed invention can also be applied toa display device for monochrome display.

In this embodiment, structures including a liquid crystal element and anEL element as display elements are described with reference to FIGS. 43Aand 43B and FIGS. 44A and 44B. Note that FIGS. 43A and 43B arecross-sectional views along the dashed-dotted line Q-R shown in FIG. 42and each show a structure including a liquid crystal element as adisplay element, whereas FIGS. 44A and 44B are cross-sectional viewsalong the dashed-dotted line Q-R shown in FIG. 42 and each show astructure including an EL element as a display element.

FIG. 43A and FIG. 44A show the display device 700 with high mechanicalstrength in which glass or the like is used for the first substrate 701and the second substrate 705. FIG. 43B and FIG. 44B show a flexibledisplay device 700 a in which plastic or the like is used for the firstsubstrate 701 and the second substrate 705. The first substrate 701 isfixed, with an adhesive 720, to an insulating film 719 on whichtransistors 750 and 752 and a capacitor 790 are formed. The secondsubstrate 705 is fixed, with an adhesive 740, to an insulating film 739on which a coloring film 736, a light-blocking film 738, and the likeare formed.

Common portions between FIGS. 43A and 43B and FIGS. 44A and 44B aredescribed first, and then different portions are described.

<Common Portions in Display Devices>

The display devices 700 and 700 a illustrated in FIGS. 43A and 43B andFIGS. 44A and 44B each include a lead wiring portion 711, the pixelportion 702, the source driver circuit portion 704, and the FPC terminalportion 708. Note that the lead wiring portion 711 includes the signalline 710. The pixel portion 702 includes the transistor 750 and thecapacitor 790. The source driver circuit portion 704 includes thetransistor 752.

Any of the structures of the transistors described in the aboveembodiments can be applied to the transistors 750 and 752 asappropriate.

The transistors used in this embodiment each include an oxidesemiconductor film which is highly purified and in which formation ofoxygen vacancies is suppressed. In the transistor, the current in an offstate (off-state current) can be made small. Accordingly, an electricalsignal such as an image signal can be held for a longer period, and awriting interval can be set longer in an on state. Accordingly,frequency of refresh operation can be reduced, which leads to an effectof suppressing power consumption.

In addition, the transistor used in this embodiment can have relativelyhigh field-effect mobility and thus is capable of high speed operation.For example, with such a transistor which can operate at high speed usedfor a liquid crystal display device, a switching transistor in a pixelportion and a driver transistor in a driver circuit portion can beformed over one substrate. That is, a semiconductor device formed usinga silicon wafer or the like is not additionally needed as a drivercircuit, by which the number of components of the semiconductor devicecan be reduced. In addition, the transistor which can operate at highspeed can be used also in the pixel portion, whereby a high-qualityimage can be provided.

In FIGS. 43A and 43B and FIGS. 44A and 44B, an insulating film 766 and aplanarization insulating film 770 are provided over the transistor 750,the transistor 752, and the capacitor 790.

The insulating film 766 can be formed using materials and methodssimilar to that of the insulating film 126 described in the aboveembodiment. The planarization insulating film 770 can be formed using aheat-resistant organic material, such as a polyimide resin, an acrylicresin, a polyimide amide resin, a benzocyclobutene resin, a polyamideresin, or an epoxy resin. Note that the planarization insulating film770 may be formed by stacking a plurality of insulating films formedfrom these materials. Alternatively, a structure without theplanarization insulating film 770 may be employed.

The signal line 710 is formed in the same process as conductive filmsfunctioning as a source electrode and a drain electrode of thetransistor 750 or 752. Note that the signal line 710 may be formed usinga conductive film functioning as a gate electrode of the transistor 750or 752. In the case where the signal line 710 is formed using a materialcontaining a copper element, signal delay or the like due to wiringresistance is reduced, which enables display on a large screen.

The FPC terminal portion 708 includes a connection electrode 760, ananisotropic conductive film 780, and the FPC 716. Note that theconnection electrode 760 is formed in the same process as conductivefilms functioning as a source electrode and a drain electrode of thetransistor 750 or 752. The connection electrode 760 is electricallyconnected to a terminal included in the FPC 716 through the anisotropicconductive film 780.

For example, a glass substrate can be used as the first substrate 701and the second substrate 705. A flexible substrate may be used as thefirst substrate 701 and the second substrate 705. Examples of theflexible substrate include a plastic substrate.

A structure 778 is provided between the first substrate 701 and thesecond substrate 705. The structure 778 is a columnar spacer obtained byselective etching of an insulating film and is provided to control thethickness (cell gap) between the first substrate 701 and the secondsubstrate 705. Alternatively, a spherical spacer may be used as thestructure 778.

Furthermore, the light-blocking film 738 functioning as a black matrix,the coloring film 736 functioning as a color filter, and an insulatingfilm 734 in contact with the light-blocking film 738 and the coloringfilm 736 are provided on the second substrate 705 side.

<Structure Example of Display Device Using Liquid Crystal Element asDisplay Element>

The display devices 700 and 700 a illustrated in FIGS. 43A and 43B eachinclude a liquid crystal element 775. The liquid crystal element 775includes a conductive film 772, a conductive film 774, and a liquidcrystal layer 776. The conductive film 774 is provided on the secondsubstrate 705 side and functions as a counter electrode. The displaydevices 700 and 700 a in FIGS. 43A and 43B are capable of displaying animage in such a manner that transmission or non-transmission iscontrolled by change in the alignment state of the liquid crystal layer776 depending on a voltage applied to the conductive film 772 and theconductive film 774.

The conductive film 772 is connected to the conductive films functioningas a source electrode and a drain electrode included in the transistor750. The conductive film 772 is formed over the planarization insulatingfilm 770 to function as a pixel electrode, i.e., one electrode of thedisplay element. The conductive film 772 has a function of a reflectiveelectrode. The display devices 700 and 700 a in FIGS. 43A and 43B arewhat is called reflective color liquid crystal display devices in whichexternal light is reflected by the conductive film 772 to display animage through the coloring film 736.

A conductive film that transmits visible light or a conductive film thatreflects visible light can be used for the conductive film 772. Forexample, a material including one kind selected from indium (In), zinc(Zn), and tin (Sn) is preferably used for the conductive film thattransmits visible light. For example, a material including aluminum orsilver may be used for the conductive film that reflects visible light.In this embodiment, the conductive film that reflects visible light isused for the conductive film 772.

Note that projections and depressions are provided in part of theplanarization insulating film 770 of the pixel portion 702 in thedisplay devices 700 and 700 a in FIGS. 43A and 43B. The projections anddepressions can be foamed in such a manner that the planarizationinsulating film 770 is formed using an organic resin film or the like,and projecting portions or depressed portions are formed on the surfaceof the organic resin film. The conductive film 772 functioning as areflective electrode is formed along the projections and depressions.Therefore, when external light is incident on the conductive film 772,the light is reflected diffusely at the surface of the conductive film772, whereby visibility can be improved.

Note that the display devices 700 and 700 a illustrated in FIGS. 43A and431:3 are reflective color liquid crystal display devices given asexamples, but a display type is not limited thereto. For example, atransmissive color liquid crystal display device in which the conductivefilm 772 is a conductive film that transmits visible light may be used.In the case of a transmissive color liquid crystal display device,projections and depressions are not necessarily provided on theplanarization insulating film 770.

Although not illustrated in FIGS. 43A and 43B, an alignment film may beprovided on a side of the conductive film 772 in contact with the liquidcrystal layer 776 and on a side of the conductive film 774 in contactwith the liquid crystal layer 776. Although not illustrated in FIGS. 43Aand 43B, an optical member (an optical substrate) and the like such as apolarizing member, a retardation member, or an anti-reflection membermay be provided as appropriate. For example, circular polarization maybe employed by using a polarizing substrate and a retardation substrate.In addition, a backlight, a sidelight, or the like may be used as alight source.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material exhibits acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on conditions.

Alternatively, in the case of employing a horizontal electric fieldmode, a liquid crystal exhibiting a blue phase for which an alignmentfilm is unnecessary may be used. A blue phase is one of liquid crystalphases, which is generated just before a cholesteric phase changes intoan isotropic phase while temperature of cholesteric liquid crystal isincreased. Since the blue phase appears only in a narrow temperaturerange, a liquid crystal composition in which several weight percent ormore of a chiral material is mixed is used for the liquid crystal layerin order to improve the temperature range. The liquid crystalcomposition which includes a liquid crystal exhibiting a blue phase anda chiral material is preferable because it has a short response time,has optical isotropy, which makes the alignment process unneeded, andhas a small viewing angle dependence. An alignment film does not need tobe provided and rubbing treatment is thus not necessary; accordingly,electrostatic discharge damage caused by the rubbing treatment can beprevented and defects and damage of the liquid crystal display device inthe manufacturing process can be reduced.

In the case where a liquid crystal element is used as the displayelement, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode,a fringe field switching (FFS) mode, an axially symmetric alignedmicro-cell (ASM) mode, an optical compensated birefringence (OCB) mode,a ferroelectric liquid crystal (FLC) mode, an anti ferroelectric liquidcrystal (AFLC) mode, or the like can be used.

Further, a normally black liquid crystal display device such as atransmissive liquid crystal display device utilizing a verticalalignment (VA) mode may also be used. There are some examples of avertical alignment mode; for example, a multi-domain vertical alignment(MVA) mode, a patterned vertical alignment (PVA) mode, an ASV mode, orthe like can be employed.

<Display Device Using Light-Emitting Element as Display Element>

The display devices 700 and 700 a illustrated in FIGS. 44A and 44B eachinclude a light-emitting element 782. The light-emitting element 782includes a conductive film 784, an EL layer 786, and a conductive film788. The display devices 700 and 700 a shown in FIGS. 44A and 44B arecapable of displaying an image by light emission from the EL layer 786included in the light-emitting element 782.

The conductive film 784 is connected to the conductive films functioningas a source electrode and a drain electrode included in the transistor750. The conductive film 784 is formed over the planarization insulatingfilm 770 to function as a pixel electrode, i.e., one electrode of thedisplay element. A conductive film which transmits visible light or aconductive film which reflects visible light can be used for theconductive film 784. The conductive film which transmits visible lightcan be formed using a material including one kind selected from indium(In), zinc (Zn), and tin (Sn), for example. The conductive film whichreflects visible light can be formed using a material including aluminumor silver, for example.

In the display devices 700 and 700 a shown in FIGS. 44A and 44B, aninsulating film 730 is provided over the planarization insulating film770 and the conductive film 784. The insulating film 730 covers part ofthe conductive film 784. Note that the light-emitting element 782 has atop emission structure. Therefore, the conductive film 788 has alight-transmitting property and transmits light emitted from the ELlayer 786. Although the top-emission structure is described as anexample in this embodiment, one embodiment of the present invention isnot limited thereto. A bottom-emission structure in which light isemitted to the conductive film 784 side, or a dual-emission structure inwhich light is emitted to both the conductive film 784 side and theconductive film 788 side may be employed.

The coloring film 736 is provided to overlap with the light-emittingelement 782, and the light-blocking film 738 is provided to overlap withthe insulating film 730 and to be included in the lead wiring portion711 and in the source driver circuit portion 704. The coloring film 736and the light-blocking film 738 are covered with the insulating film734. A space between the light-emitting element 782 and the insulatingfilm 734 is filled with a sealing film 732. Although a structure withthe coloring film 736 is described as each of the display devices 700and 700 a shown in FIGS. 44A and 44B, the structure is not limitedthereto. In the case where the EL layer 786 is formed by a separatecoloring method, the coloring film 736 is not necessarily provided.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 9

In this embodiment, one embodiment of a light-emitting device using thesemiconductor device of one embodiment of the present invention isdescribed. Note that in this embodiment, a structure of a pixel portionof a light-emitting device is described with reference to FIG. 45.

In FIG. 45, a plurality of FETs 500 is formed over a first substrate502, and each of the FETs 500 is electrically connected to alight-emitting element (504R, 504G, 504B, or 504W). Specifically, theFET 500 is electrically connected to a first conductive film 506included in the light-emitting element. Note that the light-emittingelements (504R, 504G, 504B, and 504W) each include the first conductivefilm 506, a second conductive film 507, an EL layer 510, and a thirdconductive film 512.

Furthermore, coloring layers (514R, 514G, 514B, and 514W) are providedin positions facing the corresponding light-emitting elements (504R,504G, 504B, and 504W). Note that the coloring layers (514R, 514G, 514B,and 514W) are provided in contact with a second substrate 516.Furthermore, a sealing film 518 is provided between the first substrate502 and the second substrate 516. For example, a glass material such asa glass frit, or a resin that is curable at room temperature such as atwo-component type resin, a light curable resin, a heat-curable resin,and the like can be used for the sealing film 518.

A partition wall 508 is provided so as to cover end portions of adjacentstacks of the first conductive film 506 and the second conductive film507. A structure 509 is provided over the partition wall 508. Note thatthe first conductive film 506 has a function as a reflective electrodeand a function as an anode of the light-emitting element. The secondconductive film 507 has a function of adjusting the optical path lengthof each light-emitting element. The EL layer 510 is formed over thesecond conductive film 507, and the third conductive film 512 is formedover the EL layer 510. The third conductive film 512 has a function as asemi-transmissive and semi-reflective electrode and a function as acathode of the light-emitting element. The structure 509 is providedbetween the light-emitting element and the coloring layer and has afunction as a spacer.

The EL layer 510 can be shared by the light-emitting elements (504R,504G, 504B, and 504W). Note that each of the light-emitting elements(504R, 504G, 504B, and 504W) has a micro optical resonator (ormicrocavity) structure which allows light emitted from the EL layer 510to resonate by the first conductive film 506 and the third conductivefilm 512; thus, spectra of light with different wavelengths can benarrowed and extracted even when they include the same EL layer 510.Specifically, by adjusting the thickness of each of the secondconductive films 507 provided under the EL layer 510 in thelight-emitting element (504R, 504G, 504B, or 504W), a desired emissionspectrum can be obtained from the EL layer 510, so that light emissionwith high color purity can be obtained. Therefore, the structureillustrated in FIG. 45 does not require a process of separately formingEL layers with different colors, and facilitates achieving highresolution.

The light-emitting device illustrated in FIG. 45 includes the coloringlayer (color filter); thus, light with a desired emission spectrum canbe emitted. Therefore, by using the microcavity structure and the colorfilter in combination, light emission with higher color purity can beobtained. Specifically, the optical path length of the light-emittingelement 504R is adjusted so that red light emission is provided; redlight is emitted in the direction indicated by an arrow through thecoloring layer 514R. Furthermore, the optical path length of thelight-emitting element 504G is adjusted so that green light emission isprovided; green light is emitted in the direction indicated by an arrowthrough the coloring layer 514G. Furthermore, the optical path length ofthe light-emitting element 504B is adjusted so that blue light emissionis provided; blue light is emitted in the direction indicated by anarrow through the coloring layer 514B. Furthermore, the optical pathlength of the light-emitting element 504W is adjusted so that whitelight emission is provided; white light is emitted in the directionindicated by an arrow through the coloring layer 514W.

Note that a method for adjusting the optical path length of eachlight-emitting element is not limited thereto. For example, the opticalpath length may be adjusted by controlling the film thickness of the ELlayer 510 in each light-emitting element.

The coloring layers (514R, 514G, and 514B) may have a function oftransmitting light in a particular wavelength region. For example, a red(R) color filter for transmitting light in a red wavelength range, agreen (G) color filter for transmitting light in a green wavelengthrange, a blue (B) color filter for transmitting light in a bluewavelength range, or the like can be used. The coloring layer 514W maybe formed using an acrylic-based resin material which does not contain apigment or the like. The coloring layers (514R, 514G, 514B, and 514W)can be formed using any of various materials by a printing method, aninkjet method, an etching method using a photolithography technique, orthe like.

The first conductive film 506 can be formed using, for example, a metalfilm having high reflectivity (reflection factor of visible light is 40%to 100%, preferably 70% to 100%). The first conductive film 506 can beformed using a single layer or a stacked layer using aluminum, silver,or an alloy containing such a metal material (e.g., an alloy of silver,palladium, and copper).

The second conductive film 507 can be formed using, for example,conductive metal oxide. As the conductive metal oxide, indium oxide, tinoxide, zinc oxide, indium tin oxide (also referred to as ITO), indiumzinc oxide, or any of these metal oxide materials in which silicon oxideor tungsten oxide is contained can be used. Providing the secondconductive film 507 is preferable because the formation of an insulatingfilm between the EL layer 510 to be formed later and the firstconductive film 506 can be suppressed. Furthermore, conductive metaloxide which is used as the second conductive film 507 may be formed inlayer lower than the first conductive film 506.

The third conductive film 512 is formed using a conductive materialhaving reflectivity and a conductive material having alight-transmitting property, and visible light reflectivity of the filmis preferably 20% to 80%, more preferably 40% to 70%. As the thirdconductive film 512, for example, silver, magnesium, an alloy of such ametal material, or the like is formed to be thin (e.g., 10 nm or less),and then, conductive metal oxide which can be used for the secondconductive film 507 is formed.

The above-described light-emitting device has a structure in which lightis extracted from the second substrate 516 side (a top emissionstructure), but may have a structure in which light is extracted fromthe first substrate 501 side where the FETs 500 are formed (a bottomemission structure) or a structure in which light is extracted from boththe first substrate 501 side and the second substrate 516 side (a dualemission structure). In the case of the bottom emission structure, thecoloring layers (514R, 514G, 514B, and 514W) may be formed under thefirst conductive film 506. Note that a light-transmitting substrate maybe used for the substrate through which light is transmitted, and alight-transmitting substrate and a light-blocking substrate may be usedfor the substrate through which light is not transmitted.

In FIG. 45, the structure in which the light-emitting elements emitlight of red (R), green (G), blue (B), and white (W) is illustrated asan example. However, a structure is not limited thereto. For example, astructure in which the light-emitting elements emit light of red (R),green (G), and blue (B) may be used.

Embodiment 10

In this embodiment, a display module and electronic devices that can beformed using a semiconductor device of one embodiment of the presentinvention are described with reference to FIG. 46 and FIGS. 47A to 47G.

In a display module 8000 illustrated in FIG. 46, a touch panel 8004connected to an FPC 8003, a display panel 8006 connected to an FPC 8005,a backlight 8007, a frame 8009, a printed board 8010, and a battery 8011are provided between an upper cover 8001 and a lower cover 8002.

The semiconductor device of one embodiment of the present invention canbe used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and can be formed to overlap with the display panel 8006. Acounter substrate (sealing substrate) of the display panel 8006 can havea touch panel function. A photosensor may be provided in each pixel ofthe display panel 8006 to form an optical touch panel.

The backlight 8007 includes a light source 8008. Note that although astructure in which the light sources 8008 are provided over thebacklight 8007 is illustrated in FIG. 46, one embodiment of the presentinvention is not limited to this structure. For example, a structure inwhich the light source 8008 is provided at an end portion of thebacklight 8007 and a light diffusion plate is further provided may beemployed. Note that the backlight 8007 need not be provided in the casewhere a self-luminous light-emitting element such as an organic ELelement is used or in the case where a reflective panel or the like isemployed.

The frame 8009 protects the display panel 8006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 may function asa radiator plate.

The printed board 8010 is provided with a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or a power source using thebattery 8011 provided separately may be used. The battery 8011 can beomitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

FIGS. 47A to 47D illustrate electronic devices. These electronic devicescan include a housing 600, a display portion 601, a speaker 603, an LEDlamp 604, operation keys 605 (including a power switch or an operationswitch), a connection terminal 606, a sensor 607 (a sensor having afunction of measuring or sensing force, displacement, position, speed,acceleration, angular velocity, rotational frequency, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared ray), amicrophone 608, and the like.

FIG. 47A illustrates a mobile computer that can include a switch 609, aninfrared port 620, and the like in addition to the above components.FIG. 4713 illustrates a portable image reproducing device (e.g., a DVDplayer) that is provided with a memory medium and can include a seconddisplay portion 602, a memory medium reading portion 621, and the likein addition to the above components. FIG. 47C illustrates a televisionreceiver that can include a tuner, an image processing portion, and thelike in addition to the above components. FIG. 47D illustrates aportable television receiver that can include a charger 627 capable oftransmitting and receiving signals, and the like in addition to theabove components.

FIGS. 47E to 47G illustrate a foldable portable information terminal610. FIG. 47E illustrates the portable information terminal 610 that isopened. FIG. 47F illustrates the portable information terminal 610 thatis being opened or being folded. FIG. 47G illustrates the portableinformation terminal 610 that is folded. The portable informationterminal 610 is highly portable when folded. When the portableinformation terminal 610 is opened, a seamless large display regionprovides high browsability.

A display portion 612 is supported by three housings 615 joined togetherby hinges 613. By folding the portable information terminal 610 at aconnection portion between two housings 615 with the hinges 613, theportable information terminal 610 can be reversibly changed in shapefrom an opened state to a folded state. A display device according toone embodiment of the present invention can be used for the displayportion 612. For example, a display device that can be bent with aradius of curvature of greater than or equal to 1 mm and less than orequal to 150 mm can be used.

The electronic devices illustrated in FIGS. 47A to 47G can have avariety of functions, for example, a function of displaying a variety ofdata (a still image, a moving image, a text image, and the like) on thedisplay portion, a touch panel function, a function of displaying acalendar, date, time, and the like, a function of controlling a processwith a variety of software (programs), a wireless communicationfunction, a function of being connected to a variety of computernetworks with a wireless communication function, a function oftransmitting and receiving a variety of data with a wirelesscommunication function, a function of reading a program or data storedin a memory medium and displaying the program or data on the displayportion, and the like. Furthermore, the electronic device including aplurality of display portions can have a function of displaying imagedata mainly on one display portion while displaying text data on anotherdisplay portion, a function of displaying a three-dimensional image bydisplaying images on a plurality of display portions with a parallaxtaken into account, or the like. Furthermore, the electronic deviceincluding an image receiving portion can have a function of shooting astill image, a function of taking a moving image, a function ofautomatically or manually correcting a shot image, a function of storinga shot image in a memory medium (an external memory medium or a memorymedium incorporated in the camera), a function of displaying a shotimage on the display portion, or the like. Note that functions that canbe provided for the electronic devices illustrated in FIGS. 47A to 47Gare not limited thereto, and the electronic devices can have a varietyof functions.

The electronic devices described in this embodiment each include thedisplay portion for displaying some sort of data. Note that thesemiconductor device of one embodiment of the present invention can alsobe used for an electronic device which does not have a display portion.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

This application is based on Japanese Patent Application serial no.2014-022872 filed with Japan Patent Office on Feb. 7, 2014, JapanesePatent Application serial no. 2014-022871 filed with Japan Patent Officeon Feb. 7, 2014, Japanese Patent Application serial no. 2014-051131filed with Japan Patent Office on Mar. 14, 2014, and Japanese PatentApplication serial no. 2014-051136 filed with Japan Patent Office onMar. 14, 2014, the entire contents of which are hereby incorporated byreference.

What is claimed is:
 1. A semiconductor device comprising: a firsttransistor and a second transistor each over a first insulating thefirst transistor comprising: a first oxide semiconductor film; a secondoxide semiconductor film on and in contact with the first oxidesemiconductor film; a third oxide semiconductor film on and in contactwith a top surface of the second oxide semiconductor film and sidesurfaces of the first oxide semiconductor film and the second oxidesemiconductor film; a first gate insulating film on and in contact withthe third oxide semiconductor film; and a first gate electrode over thefirst gate insulating film; and the second transistor comprising: afourth oxide semiconductor film; a second gate insulating film on and incontact with the fourth oxide semiconductor film; and a second gateelectrode over the second gate insulating film, wherein each of thefirst oxide semiconductor film, the second oxide semiconductor film, thethird oxide semiconductor film, and the fourth oxide semiconductor filmcomprises indium, gallium, and zinc, and wherein, in each of the thirdoxide semiconductor film and the fourth oxide semiconductor film, aproportion of indium atoms is lower than or equal to a proportion ofgallium atoms.
 2. The semiconductor device according to claim 1, whereinthe first transistor comprises a source electrode and a drain electrodeeach on and in contact with the third oxide semiconductor film.
 3. Thesemiconductor device according to claim 1, comprising a secondinsulating film over the first gate electrode and the second gateelectrode, wherein the first transistor comprises a first sourceelectrode and a first drain electrode each over the second insulatingfilm, and wherein the second transistor comprises a second sourceelectrode and a second drain electrode each over the second insulatingfilm.
 4. The semiconductor device according to claim 1, comprising asecond insulating film over the first gate electrode and the second gateelectrode and in contact with the third oxide semiconductor film and thefourth oxide semiconductor film, wherein the second insulating film is anitride insulating film.
 5. The semiconductor device according to claim1, wherein the third oxide semiconductor film covers the first oxidesemiconductor film and the second oxide semiconductor film.
 6. Thesemiconductor device according to claim 1, wherein, in the second oxidesemiconductor film, a proportion of indium atoms is higher than aproportion of gallium atoms.
 7. A display device comprising thesemiconductor device according to claim 1, wherein the display devicecomprises: a driver circuit portion comprising the first transistor; anda pixel portion comprising: the second transistor; and a display elementelectrically connected to the second transistor.
 8. The display deviceaccording to claim 7, wherein the display element is a liquid crystalelement.
 9. The display device according to claim 7, wherein the displayelement is an electroluminescent element.
 10. The semiconductor deviceaccording to claim 1, wherein the first gate electrode is electricallyconnected to one of a source electrode and a drain electrode of thesecond transistor.